How do I deal with this?

I (19) feel terrible about it. But lately I’ve been kinda comparing myself to my younger sister. It’s mostly because of something that happened a couple months back, we were hanging out at my ex’s house, and they were sitting near each other and i got the vibe that they were kinda flirting, I asked my sister about it and she said no and she got upset that I thought she would do that to me. We had a talk about it and we were okay after, I trust that she wasn’t trying to. I’m not so sure about him, but he kinda just acts that way so maybe he wasn’t flirting idk. But it made me feel really insecure, I know he’s my ex, but it made me question what if he thinks she’s better or more attractive or prettier? And it made me feel awful and anxious. And times after that when we’ve hung out I’ve been afraid of the idea of him liking my sister, and it makes me feel really anxious, and not good enough. I don’t wanna feel that way about my sister, and I don’t want to allow my ex to constantly make me feel insecure, he shouldn’t still be able to do that and I know. How do I handle this? I keep connecting pieces of “evidence” in my head and it’s making kinda spiral and feel all the more insecure.