<human>: Could you distill the essence of the text into 1-3 sentences?

So, my boyfriend (29) of 2.5 years and I (26) broke up 6 months ago. It was a pretty mutual breakup, both of us agreeing things had run it's course. It was a little awkward because his sister and I are really good friends, but all things considered we managed to make it work and avoid things being too complicated. 

Yesterday, one of my best friends (26) told me she thinks she might be developing feelings for my ex. She said she'd been hanging out with his sister and in turn has been hanging out with him, and I guess she now has a bit of a crush. Recently I had noticed them talking and hanging out without his sister around, so I wasn't really entirely shocked. 

Initially I thought I was fine with it, considering I have no desire to get back together with him. I've been going on dates and seeing other people, so it seemed silly to be upset about it. However after talking to a few friends and thinking about things for the past 24 hours, I feel like I'm less comfortable with the idea. Is it worth telling her I'm not okay with it, even though I'm over him? I just don't want our friendship to change, but I feel like there's no way it wouldn't, and I'm worried that even knowing that she'd consider dating him makes me question our friendship. It's just there are billions of other guys out there, does she really have to date my ex?
<bot>: Am I just being petty, or am I justified in asking one of my best friends not to pursue my ex even though I no longer have feelings for him?