I touched my stepmoms butt and she didn’t react what should I do?

Go for it !

That's super creepy. Have you seen all these news stories lately about sexual harassment and sexual assault going on? They're all guys doing exactly what you did. A woman feels threatened and doesn't know what to do, so she pretends it's not happening and in so doing hopes it just stops. You're either imagining that smile or it was a grimace, an expression of her being made nervous but not knowing what to do because she was afraid of the huge blowout and all the negative consequences that would come from acknowledging it, including between her and your dad because it would put your dad in the untenable position of having to choose sides--hers or yours. So unless your goal is to become a pervy social reprobate like Harvey Weinstein that's a total sleaze and everyone looks at with stomach-turning disgust, knock it off. It's not cute. Nobody thinks it's cute except creepy you.

Its your choice to feel her butt.
But if your dad found out,what would happen.

Frack 'er blue.

i wouldnt do it again, that kind of stuff can get you into trouble, just being honest