Sister hanging out with my ex

I found out recently that my sister has been hanging out with my recent ex. I’m worried they’re gonna end up dating and I’m gonna feel horrible. While he and I were dating she considered him “annoying and immature” and never wanted to spend time with us. Now that we broke up she said “he’s actually really caring” and “a really good friend”

I’ve talked to her about it but not much came of it. She still spends time with him and it makes me question whether I’m being selfish?