I really don't want my sister meeting up with some creep she met online to paint.
What will happen if you don't allow your sister to bring over a guy?
She will raise hell. That's for sure.
If it is your home, then it is your rules. Your sister can pout all she wants, but that won't change.
My sister won't get it and she'll give me the silent treatment. I think I'm going to lose my shit.
Just remember if she is angry with you you are doing the right thing. You need to protect her from strangers even if she doesn't understand right now.