Didn't Recognize my Ex-Wife's sister and asked her out on a date

I'm at the super market in my home town. Haven't been back in seven years. Coincidentally, that's when the divorce happened with my ex; let's call her Tammy. Anyway, I'm buying a watermelon and I see this cute girl who is also browsing the same fruits. Looking back at it, she seemed familiar, but, being the romantic I am, I believed I was simply astonished at her looks. So I ask her if she needs help picking out a watermelon. She's always been the forgetful type, and along with that I only saw her once before (she lived in Hong Kong), so I'm not shocked that she didn't go "oh, hey u/LuciusDickusMaximus, haven't seen you in ages." No, she just treats me like I was a stranger, so I think, bingo. I see no ring, so I keep flirting and eventually, I asked her if she has any plans for the evening, and she invites me to 'game night.' I agree (reluctantly because the last game night I had was with Tammy's family), and that night I go to the address she provided. Knocked on the door, was greeted by my ex-father-in-law and a shocked face that said "What the hell are you doing here?" I immediately ran and since then I pieced together (with the help of Facebook) the family tree.

It's pretty funny to think about now, but it was so cringy and I can't imagine how they must have cursed my name.

TL;DR: Ran into my ex-wife's sister at the market, and, not knowing it was her, asked her out and then found out at the doorstep what vile people she was related to.