Guy trying to date both me and my sister?

so long story short me and this guy, I’ll call him J, have known each other since 2015 and been on an off a lot since then.

Just a little back story, I have a sister, I’ll call her b, and honestly she very often goes after guys that me and my older sister have dated before. I do think it’s weird, I would never do that but that’s just because I would always feel like the second choice when the first sister didn’t work out. And if she doesn’t feel that way then that’s great. just my opinion, other people might think it’s not weird but regardless I would never tell her she can’t go out with a guy I’ve dated I just think it’s weird. But what is even weirder, to me, is that a lot of guys have tried to get with me and my sister either at the same time or a couple months after. Last time this happened she told me the guy i was talking to was hitting her up too and i cut him off.

So back to this situation, the first time I met J in person my sister and a couple of my friends came with because I didn’t want to meet him alone. He knows she is my sister. And ever since then whenever me and him were “off” she would talk to me about how she thought he was really attractive and wanted to go out with him but that just never happened.

So fast forward to last year 2022, me and j started talking again. (I say talk because we never were exclusive we just got really close again) me and j were talking for about 4 or 5 months and he said he wanted to be with me and all that. I was going through a lot at the time with having a 4 year old, a friends death and my job that left little time to have friends or a relationship. at some point we just kind of stopped talking. A week after we last talked, he followed my sister on Instagram. And now they’re going out.

I never told my sister that me and him were involved a couple months ago so she has no idea and I am NOT mad at her at all for going out with him now. I think I’ve just kind of realized that he isn’t a nice genuine guy I thought he was this whole time. I think he is one of those weird guys that tries to get with both me and my sister. And she doesn’t know that and I don’t know if I should tell her that he moved on from me to her in a week.

Again, some of you might not think this is weird and that’s okay. But having lived with guys trying to get with both me and my sister like we’re trophies to collect is really exhausting and upsetting. I would want to know but maybe she wouldn’t. Do I tell her, or do I just let it play out?