Alright some Sheik advice.
1. Never fight Sheik on/near the ledge if you don't have to. Falcon's recovery sucks, and while it won't kill you at low percents, Sheik will rack up a ton of damage on you.
2. If Sheik is ledge stalling don't try and take it from her unless she is just double jump re-grabbing it. Stay at a safe, but threatening distance and Dash Dance (if you are on BF, be near the side of the side platform closest to the center). If you want to feel a bit more threatening, turn your back to the ledge and start Wave dashing/Wave landing toward it like you are going to take it. Remember you have 8 minutes on that clock. That's a long time to up-b stall without messing up, especially under pressure.
3. Never let Sheik camp you if you have the lead. Backoff, force her to fight you mid stage. If she is needle camping just move out of her range.
4. When you grab her at 30%, you can start up-throw up-airing. At 60% even the best DI will lead to at least a double up-air, and possibly a knee. 70-90% is your golden opportunity. All DIs from up-throw lead to knee. After that up-throw up-air.
5. Always hold ledge when edge guarding her, unless you mess up your invincibility and her fire is going to hit you. Then either do standard get up or ledge attack. If she lands on stage, stomp at low to mid percents, knee at higher percents. Try to get a reverse knee.
6. Sheik loses to Falcon in the air. Don't let her bully you with her crappy back-air and forward-air. Challenge her jumps with up-air. If she starts doing full hop needles, use Falcon's amazing air mobility and full hop knee her.
7. The only thing Shiek has on you at lower percents is grab. It's all she wants. You can cc Dash attack until 72% when it's fresh and cc f tilt until 93%. Punish those with grabs.
8. Shiek, compared to Falcon, is slow. If you have ever played the match up from the other side, you realize how hard it is to actually catch Falcon. Shiek relies a lot on Falcon giving her all the rope she needs to hang him with when he approaches. Abuse your movement. Shiek does not camp Falcon because it is easy, she does it because she can't fight him aggressively and win.
9. Get fast out of shield. A lot of what Shiek hits your shield with is not safe, and getting fast at stomping down-smashes on shield and neutral/up-airing badly spaced tilts.
10. Your counterpick of choice is FD. Her's is Yoshi Story. Yoshis ****s your grab combos on her up super bad. They are the perfect height to make guaranteed death into a 50/50.
11. Your neutral will either be Dreamland and, if Shiek is smart, FoD. If she gives you Battle Field, go Battle Field. However you should really learn how you can abuse FoD, and how safe it makes stomps.
12. Once Shiek is mid stage you have several options for approach. DD Grab, well spaced(read tippy toe) neutral-airs, and empty hop short hop aerials to bait shield grabs. There are others, but keep keep to the golden rule:
13. Don't get grabbed. Seriously, make this a focus. So much of Shiek's offense shuts down when she can't grab you. Again you are waaaaaaay faster than Shiek. She has a real hard time grabbing you unless you go in. Build your offensive options around that.