As a father of a now adult child, I saw both sides of that. In the end you cannot be seen to be too invested in your child’s boyfriend, fiancé, whatever. If you like them too much, that could make them decide to break up. If you hate them, it may cause your child to to stay with them as an act of rebellion. In the end, the best you can do as a parent is to hopefully teach your children good values and hope they make good decisions. As a now grown child the best you can do is to think clearly about your choices and who you associate with. Eventually the “fun” passes and it is character that remains. OP make you decisions based on what is best for you and your future.

It's very weird that your little brother treats your boyfriend like a son. I couldn't get past that.

Just have a fix the relationship baby, you've got this.

better to get out now then later down the road when they start expecting a marriage