I can't help but think that some of this is "outcome dependent", coupled with "rationalization".
If your wife catches you with the waitress, it's "flirting". If the wife doesn't and the waitress ends up telling you to fuck off, it's "flirting". If the wife doesn't and the waitress ends up blowing you in the back, it's "hitting on".
I tend to talk to anyone at any time. I suppose if that "anyone" is a woman, it's "flirting". I don't know. It's rarely ever with the intent of "I'ma gonna bang this broad." I don't have that skillset. Now that I'm married, I don't cheat, I'm good like that, but then again, I don't have 1000 opportunities either, and I can't honestly say that it never crosses my mind whether the deal could be closed under different circumstances. Is that flirting? I don't know. I tend to think it is, but see above. "Rationalization".
But as an aside, I don't understand the OP; were you flirty with your (step) mother? Or some girl who happens to be engaged, and who's mom happened to be your (step) mother's boss? Where does your sister fit in? For the love of god who are you flirting with?