What to do when you fancy the same guy as your sis?

Updates
10 mo
Actually I don't know if she's really into him or what their status is but I just have the impression there might be more going on because they joke around a lot in a flirty way on social media and I know she stayed a few times at his place but maybe they're just regular friends and I'm overthinking it.
Of course I tried to ask her about it but she didn't give a clear answer
Updates
10 mo
And I also don't know if he's actually interested in me or just playing around. We chat a lot every day and sometimes hang out together but haven't been on any proper dates yet. Recently we had sex for the first time which probably was a mistake but can't change it anymore
Updates
10 mo
I asked again and turned out that they have only been friends with benefits until recently no feelings involved
What to do when you fancy the same guy as your sis?
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