Sheik's weakness is somewhat irrelevant. Player experience, reading comprehension, technical skill level and ability to stay clear minded/unaffected is what matters.
If you really want to know some fundamental weaknesses, here are a few I can think of:
Sheik's grab game is ridiculous therefor she will go for alot of grab attempts. A missed grab has alot of lag. If you can dodge grab attempts then you shold have a good punish opportunity. Good Sheiks knows when to grab, which is often off of a tech chase or a punish in which case you can't dodge the grab.
Sheik wants you to mess up so she can punish you. If your spacing is on point, she will have a hard time getting much started.
Try and keep calm and be patient. Don't try and do new technical stuff with the risk of messing up as you will get punished.
This is starting to derail. sorry for that.
I think your best bet is to look for your opponents weaknesses and exploit them because let's face it, Sheik is a beast, top tier character.