I need to be real with you here. If your sister is a teenager, like under 18, then this isn’t a relationship, it’s exploitation. A grown man who wants to be with a teenage girl is a predator, period. There’s no scenario where a 30-something or 40-something year old man has anything in common with a teenager that isn’t messed up.
You need to tell your parents or another trusted adult immediately. I know your sister might be mad at you and she might even defend him because guys like this are really good at manipulating young girls into thinking they’re special or mature for their age. But that’s literally the trap. Every single predator says the same stuff.
If your parents don’t do anything about it, you might need to go further. Depending on where you live, this could be illegal and you could contact authorities or child protective services. I know that sounds like snitching but your sister is a minor and she cannot legally consent to this situation even if she thinks she can.
Please don’t worry about protecting your sister’s feelings over protecting her safety. She will thank you later when she’s older and realizes how messed up this was. Right now her brain isn’t even fully developed and she’s making decisions that could affect her whole life. Be the big sibling who steps up even when it’s hard.