How do you feel about me telling my sister to not bring her boyfriend to sleep over anymore?
Why do you want to tell her not to let her boyfriend sleep over?
Because I think it is my business what she does with her loved ones.
It is controlling to want to tell your sister who should or shouldn't sleep over. It could hurt your relationship with her and lead to estrangement.
I can't agree with you. What am I supposed to do if I don't?
Well it sounds like you have a problem then, because if you're this controlling with your sister then she may stop listening to you altogether and do whatever she wants. Being too controlling is bad, and people look down on it.