As Sheik, it doesn't really look like you have a move that covers everything, so you'll have to wait until the tether character pulls up, and react to the direction they're drifting (also called strafing so forgive me if I use that term sometimes) to.
There are 3 possible drifts, all the others in between being vastly inferior for many reasons.
-Towards the center of the stage : Tether users typically opt for this one when they only have one tether left and no more jumps/bumps to help them recover. The reason is, if you they drift that way and you hit them, they are going to go flying forward, towards the opposite corner of the stage. They pick this to survive. When they go for this drift, your aim should be to pop them up before they hit the ground, and then proceed to send them back offstage. You have 30 frames before they touch the ground. With Sheik, I would say waveland ftilt and ledgehop dair would do the job, and then your goal should be to fair the victim offstage again before they touch the ground. But you do have to be quick to react. If they land, consider your edgeguard opportunity wasted, but you can still stack on some percentage, and at higher % ranges land a kill move. After they land, they go through a 20 frames long landing animation. With Sheik, I would go for a waveland into a pivot grab if I wanted to stack on some damage, or a waveland into a charged usmash if I wanted to land a kill. But you know your character better than I do, you may be able to come up with better answers.
-Away from the stage : This is a mixup designed to avoid the followups from above. But you have 50 frames to react to this, so unless you're choking hard you should be able to get a followup every time. What followup to go for... I would say walljump fair if you can, reverse fair if you can't and you're feeling fancy, or sweetspot bair if you're a lame weabo. Tether users won't go for this one nearly as often, because if they get hit, they're offstage already, they'll fly towards the closest blastzone relative to the ledge, and they won't recover their tether. It's a pretty bad position to be in. I see many new players just wait and sleep on the ledge when they see their opponent strafe away from the stage - that's a mistake. That's the same as going for a footstool after a jab reset : it may look like the most meta followup ever, but there is zero reason to ever go for this when you can actually land a hit.
-Towards the ledge : This third, much less common option at low level, consists in edge-cancelling the 20 frames long landing animation so that you "only" have 30 frames to punish the tether user instead of 50. It is also sometimes used to avoid some waveland-based followups such as waveland pivot grab. If the tether user goes for this, you pretty much have to go for a ledgehop dair or a ledgehop reverse fair, as Sheik. Anything slower won't cut it.
It's not nearly as precise as this, and it depends on the player, but at top level I would expect about 50% edge cancels, 30% center of stage and 20% away from the stage for the first tether pull, and 30% edge cancels, 60% center of stage, 10% away from the stage for the second tether pull without touching the floor.
However, all of that is assuming you have managed to edgehog the tether character. Against ZSS and Ivysaur, getting to actually set up that edgehog is what you should focus on. Those characters' recoveries are designed to function and let them recover despite the braindead followups that their opponents get if they edgehog them.
I can't speak for ZSS because my understanding of the character is not as good as I'd like it to be (basically all I know is "punish her when she recovers high, until she recovers low" and "stuff the F out of her down B"), but I can give you a pretty decent rundown of how Ivysaur recovers so that you can try to figure out what to do about it.
Ivysaur's recovery is all about resources. She has a double jump, and two dair bumps (the third dair and every dair after that all share the same bump power - the first two are more powerful, so they're more important to her survival). Keep track of those resources. The more you make her spend them, the better. The question should be, how do you make her spend them. The two main ways are to threaten her with projectiles, and jumping at her when she's offstage. I know Sheik can do at least one of those reliably.
Why are those resources important ? That's a good question, reader, I'm glad you came up with it. The answer is going to be a bit long but bare with me.
See, the thing is, Ivysaur has two projectiles : Razor Leaf (side B) and Seed Bomb (down B). And both of those are most effective after a delay.
RL decelerates as time goes on, so for the last half second or so that it is onscreen, it will be almost stationary and will hit three or four time that particular area of the screen. It just so happens that half a second is the amount of invincibility you are granted for grabbing the ledge. It doesn't take a master's degree in mathematics to understand that simply holding the ledge for days isn't going to net you that precious edgehog that you desire so much. And by the time the last hit is done, Ivysaur can already be on her way to the ledge. There are two ways to deal with this : the first one is to be frame tight and grab the ledge right before RL's hitbox becomes a threat, so that you're still invincible by the time it disappears. That's where the resources come into play : if Ivy had her dair bumps, she was able to save her double jump until that moment, and instead of going for the standard razor leaf - upB, she'll go for razor leaf into double jump fair, which will hit after the invincibility runs off, and with a pretty good range too. The second option is to go for the edgehog sooner than you need to, and then dropping from the ledge before the invincibility runs off, before the razor leaf disappears, and before Ivysaur is even able to upB to the ledge. And then you go after Ivysaur herself rather than waiting for the edgehog. Again, if Ivy has managed to save her double jump, she'll be able to avoid that by double jumping away. Some characters such as Marth are especially good at doing this because of how their bairs cover a lot of area, it might not be Sheik's forte though.
And then there is Ivysaur's second projectile, Seed Bomb. This one hits a lot of time after we use it because, well, it has an arced trajectory. It hits so late, in fact, that it is impossible for you to edgehog Ivysaur if she does it :
-If you time your edgehog so that Ivy doesn't get to pull up her tether, she can wait until your invincibility wears off, then wait some more, and then the seed bomb will come down to blow you up. She even still has a bit of margin before she is forced to pull up.
-If you time your edgehog so that you're invincible by the time the seed bomb explodes, Ivysaur will be able to tether to the stage before you edgehog.
So what's the weakness ? Again, resources. Ivysaur can aim her seed bomb and give it an arc rather than shooting it straight up. But that arc is pretty small, so Ivy has to spend her double jump and eventually a dair or two in order to be in an area from which she'll be able to shoot the seed bomb so that it'll hit the ledge. And since that area is pretty close to the stage, it's an area most characters can threaten with aerials. Threaten that area and she'll spend the resource she needed to get there in an attempt to get as far away from it as possible.
The seed bomb recovery is most useful against characters like Marth who have the right bair for the job, so as Sheik, it's less likely that Ivysaur will go for it in the first place. And since your bair isn't as threatening, you really ought to find another way to make her waste her resources, or at least train so that you can do the frame tight edgehog consistently under the threat of a Razor Leaf.
Ivysaur's recovery requires a lot more interaction than it looks like it does at first glance, nobody's going to laugh at you for struggling with it. It is a good recovery - a particularly polarizing and awfully designed one, but it does get the job done pretty consistently.
You have enough ledge invincibility that you don't have to worry about projectiles from ivy or zss. Shino-stalling makes this even more true.
I just crunched the numbers and this might be a really good idea to deal with Ivysaur's Razor Leaf recovery :
Ivysaur's Razor Leaf is out on frame 18, she can then act on frame 42. Then it takes her 21 frames to tether the ledge and pull to it, at which point Razor Leaf would still be here for 11 frames.
But. It takes Sheik about 42 frames to Shino stall.
This means that you have 21 frames to react to Ivysaur's side B. It is pretty hard to identify it for the first few frames, but that's definitely more time than you need.
However, keep in mind that if Ivysaur has managed to save her double jump, she can double jump and toss a second razor leaf after her first, so you might need to do that twice against an Ivysaur who knows the trick.
