Is your sister your best friend 1

Best friend like sister ?!

Hey :)

Lately my best friend (16) has been meeting my sister (17) now and then, which is actually not bad, just instead of letting me know (16) or maybe asking me if I want to come with me or something, they always do that behind my back and meet alone while I have to get bored.
In addition, when my best friend is with me and my sister notices, they keep talking to each other and almost completely ignore me (it can also be that I'm just imagining it because of jealousy). Sometimes my sister takes my best friend into her room to "show her something" and then talks to her for half an hour in there, laughing, doing nonsense, etc. And only a few days ago it was so that my best friend was with me again and my sister just came into my room and asked her if she wanted to go out with her afterwards, whereupon I immediately said "no!" Said because it upset me and I wanted to spend the day with my best friend and not alone!

I get extremely pissed off every time I think of them!

Am I exaggerating? Am I TOO jealous, or is my jealousy justified?
And what can I do so that it all finally comes to an end?

Thanks for any reply!