You're talking about from Peach's side, right, ARC? I'm not a Peach player, but from Shiek's perspective, it's difficult to deal with mass crouch cancels. (And I mean a peach who crouch cancels everything) Since you can cc shiek's dash attack to about 70% i believe and you can cc a sheik tilt until around 85%, you really cut down sheik's options.
Condition the sheik to shield! Dash attack erratically (like dash dance and dash attack at weird timings, so you can catch the shiek in her movements and jumps) then start dash grabbing when you condition the shiek to shield. Grab is so good. Also djc aerials to grab/dsmash are VERY good. (especially if you've eaten away at her shield a bit already).
Tech chasing with fc is good since you might have trouble setting up a fc otherwise if the sheik gets in your face.
Turnip spam is obviously good.
That's about all I can give you. Probably not too helpful since you know all this but I felt like typing.