I’m not really sure how to start this. I’m in a really uncomfortable situation and would appreciate some outside perspective.

Back when I (24F) was in college, I had a serious relationship with someone I’ll call Luke (26M). We dated for a little over a year and were pretty serious. I got pregnant. It wasn’t planned, but I decided I was going to keep the baby. Luke initially said he’d support me. We agreed to tell our families and figure things out from there. The next day, he disappeared. No calls, no texts. A couple of days later, I got a package in the mail with some cash and a short note that just said something like, “For the abortion.” That was it. I never heard from him again.
I was angry, embarrassed, and honestly overwhelmed. I ended up moving back home and raising my son on my own. I didn’t tell my family who the father was. I just said the relationship didn’t work out and that I didn’t want to talk about it. They didn’t push.
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My son is two, and things are okay. A few weeks ago, my sister (28F) told us she was bringing her new boyfriend to her birthday dinner. It was Luke. We both immediately recognized each other, but neither of us said anything in front of anyone. I left the dinner early. I think I just felt stunned and unsure how to even process it.
Later that night, he came by and asked if we could talk. He told me his parents found out about the pregnancy back then, and that they pulled him out of school and cut him off from me completely. He said they were the ones who sent the money and the note. He thought I’d had an abortion and that it was over. We talked for a while, and he asked if I was going to tell my sister.

That was a few weeks ago. I haven’t told my sister anything, and Luke hasn’t reached out again. I don’t want to lie to my sister, but I also don’t want to cause drama if this relationship fizzles out on its own. At the same time, if he stays in her life long-term, it feels worse to stay quiet.
I’ve been avoiding being around them. I think she’s noticed. I don’t think she suspects anything, but I also know I can’t avoid this forever.