Am I overreacting and jealous? Is it a turn off?

So I started seeing a guy two months ago. Turns out he knew my sister before me (6 years before me) I found this out as I saw he liked her profile photo. And he liked a recent photo of her (with her husband) if I bring it up with him that he liked her photo will it give stalker vibes? I think they have mutual friends but I don’t think they have met (hope not) I really need to start thinking positive about myself.

Am I overreacting and jealous? Is it a turn off?
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