That's a very complicated topic
But basically Peach only has a few good combo breakers in her moveset that she can depend on against Sheik to get herself out of stuff
Main tools she employs are
1) Nair variants
2) Air-dodge variants
3) Float away
4) Double jump away
Umbrella usually isn't an option since Sheik tries to be below her, though it deserves some attention if you ever wind up in that situation
A lot of simple stuff like D-throw F-tilt can be followed with a variety of good punishes if you can call or respond to her defense accordingly
It is helpful to recognize how the Peach handles pressure-heavy situations, though simple reaction time or educated guesses based on what the worst/best things you could hit can suffice... though that assumes they understand the situation, so recognizing habits or reaction time is better. A lot of people don't think of this style of follow as a combo, but an extension of combat, since Peach can technically escape if she reacts/calls/whatever every time. So a lot of combat habits may surface here (mashy people might try to Nair usually, or pick options that involve inputs like air-dodges). Though you'd expect adaptive players to change anyway. But enough of that.
Man I'm tired.
If Peach is trying to move away from you, you can generally chase her down with Fairs when she's floating away. It's pretty good to do so after D-throw F-tilt. Depending on height, you can also move in and F-tilt, though this one is harder. Fairs generally apply more pressure anyway.
If she's attacking out the D-throw F-tilt, you can do a bunch of stuff. Sometimes they attack with Fairs just for the meaty hit and priority, in which case you can stuff them. Nairs can be outprioritized depending on positioning with F-tilt, which leads to awesome combos. You can also use Fair. Another underlooked (and often inferior, though it has good moments) option is D-smash. The invincibility eats her Nair, often sneaks under Fair, and lasts long enough to often trump air-dodges intending to ground her. Can be useful to put her on the ledge, or offstage, for early percent stage advantage. Peach is AWFUL on the ledge, and putting her there is very good. You can make your "combo" last a stock by simply grinding her from that position if you respond to her actions appropriately. Her lack of a good ledgehop is a very abusable thing.
I'll add more later.
