haha, this actually took me sometime to answer and sadly I'm missing one critical piece of information so I can't give you a 100% guaranteed correct answer, anyways.
To do a ftilt into reverse dsmash it can be a true combo but not until the much higher percents so although it is a good mix up that can cause damage if your opponent doesn't react fast enough after being ftilt it is possible to be escaped unless you wish to wait till the 70's more or less depending on the character weight
You see the cap that sheik ftilt can grant sheik a advantage is 14 frames and sadly the first hit of the back hit of the dsmash is frame 14. So even with buffering it's only mildly possible to do on every character, however every character who isn't marth this could work even better on them.
Air dodging in brawl takes 3 frames, attack at it's fastest in the air take 2 frames so we can cut down the frame advantage needed to preform this on those characters because of this. So instead of needing 14 frames we could argue for 12 frames or 11. We also can account for human reflexes not being perfect which could sway the numbers even more to are advantage not to mention people can't hit the buttons every single frame in succession. There's also the 1 frame of input time that the game it self needs to read the command the put in. We do not have to worry about this because of buffing that we have near the end of the ftilt will make the dsmash come out frame perfect on the first possible frame.
..Also although it can be escaped in the low percents it also might be faintly possible as well since the time someone is traveling in the air when hit in the very low percents is very small and plus the fact that when a characters lands they will be unable to do anything for a few frames including shielding so it might be possible to catch someone off guard and have it work them as well, but not guaranteed.
Sadly, like I said about there's one critical piece of information I do not have and that's the frame data on how many frames it takes for sheik to turn around and does the reverse or any smash for that matter count these frames or just bypass them when doing a reverse smash or attack.