There are two things about an aggressive Pikachu that you have to keep in mind:
-Pikachu's Hurtbox
-Pikachu's Up Tilt
Punishing a Pikachu that is landing with an aerial becomes increasingly difficult due to the hurtbox being very close to the ground in relativity to other characters. What this means is that you should attempt to punish (if you are still grounded) with a F-Tilt or D-Tilt to guarantee that you'll at least hit Pikachu. At lower % (Starting around maybe 8%) F-Tilt will lead into a grab (F-Throw -> Fairs / Bouncing Fish) and from there you can press your lead by spacing with retreating Fair into Grab/Bouncing Fish. Pikachu doesn't have amazing Frame data so you can abuse that with Sheik's speed and her own Frame data.
The next thing is what generally will give Pikachu the lead: Landing an Up Tilt. Ways that this can happen could be an ill spaced F-air or a more common thing is Pikachu rolling towards you and starting it from that. Ways to avoid this is try to stay away from Pikachu while He/She is on the ground/Shielded. You can try to bait that out with tomahawks (Short hop Fast Fall grab) since the Pikachu could be trying to do an up-tilt to punish you.
This is about all I have for now. Hope it helps.