She gets edgeguarded hard, the has lackluster options from above (not that this couldn't be said for most characters), her moveset doesn't lend itself well to approaching, and thus many of her good options lose to shield. For example, many Sheiks like to ftilt to stuff Fox's approaches. If she does this, you can shield it and punish (but don't wait, act quickly).
It's not a tiny window, but you should try to act immediately. Usmash OoS is pretty much guaranteed regardless of spacing. Alternatively, you can just dash dance, which also beats out any attempts by the Sheik to pressure as well, but you don't always have room to do this comfortably, and Sheik has needles.
Although Sheik's moveset is generally amazing, many of her options are actually outranged by your equivalents, not that you should use them necessarily. What you can do is try to wall her out. Bair is really good for this, but despite Sheik's limited dash dance, she can maneuver around this and get a grab.
Aside from edgeguarding, waveshine is reliable and should be comboed into up something or dsmash usually, be sure to keep shine unstale though (pummel before throws at higher percents, or just do fewer waveshines before starting a juggle). Try to keep aerials low to the ground. You probably shouldn't go for uthrow uair at zero, you can do some utilts or usmash into a bair.
Just focus on putting hitboxes between you and her that push her away if you don't want to be grabbed, and otherwise keeping pressure tight, and movement difficult to track.
Also, nair to nair to nair is the combo that works on literally every character after some percent. Once it starts to knock down, it forces a tech-chase or edgeguard when she DIs so you can't directly combo.
Sheik's OoS game is also bad from behind, like most characters. If you condition her not to challenge your pressure, you can probably expect a wavedash or roll away if you drill the back of her shield.
