I'm sorry to tell you, and not to offend you, but you do not own the house.
You do not pay the money for the house, your parents have to pay the bill. If they don't complain about it, then you cannot do anything about it.
Only the parents, or your sister herself can get the boyfriend to leave, but besides that you cannot do anything about it.
It is not your personal decision to make.
If I was your sister I'd be offended, by you complaining about my boyfriend.
It is not your personal business, unless he physically assaults your sister or a bad violent argument ensues.
You shouldn't be worried about anything unless the relationship becomes violent.
Like I said if i was your sister, I would be offended at you complaining about my boyfriend.