Definitely not safe. The first hit to second hit allows for a nice mixup (a frame trap, really) since there are only 5 actionable frames between the hits for your opponent (only enough to roll, spot dodge, or jump), but Shiek can definitely wavedash out and punish you if you use the second hit. It has -28 frame advantage. Wavedash grab takes 21 frames. If you use the first hit and then just do nothing, you can sometimes get away, but overall it is risky.Sheik is going to be abusing her shield to try to get you to do something laggy. You can throw a boomerang into her shield and it will bounce back to you immediately, ready to be thrown again. If she is being the aggressor, she shouldn't be able to counter a SH retreating boomerang throw. So that is going to be your staple to keeping her out. Jab is also a godlike tool that will stuff her from very far out. If she short hops to try to space on your shield, a turn around up-tilt will deny her cleanly.
As long as you don't let Sheik in with her grabs, you'll be in good shape. Try to throw out safe moves. For some reason, double hit f-smash feels pretty safe on shield (you can spotdodge or roll if she tries to punish it). Maybe I'm wrong though.
Against Shiek, I recommend using platforms a lot - not necessarily camping them, but just as a way to move around. It forces a lot more fighting in the air where Link is very strong. Shiek's aerials also lose badly to crouch cancel, so it is an overall good way to play. Don't go in too hard against Shiek since you don't really have to and Shiek has a pretty hard time going in against Link.