I think u were an ass
You're divorced, right? She'd have no power over you. At best, it'd be the other way round, but both of you could deny anything happening. She could get jealous, upset or difficult but it's unlikely and you won't see her that much anyway. Also, if for some reason your sex had caused a breakup of their marriage, she won't be the aggrieved party and stepdad will go away, presuming they got married when your DIL was grown up. Would've been riskier if she'd been the stepmom.
If you'd just been polite and nice I think you could've fucked her at least once with no consequences.