Should I let my non muslim sister and her non muslim boyfriend stay at my home?

I'm a muslim, so is my wife and our children. My parents are non muslims and so is my sister and her boyfriend. The plan was for my parents and sister to stop off at my home for a week then go to visit a friend for a few days further down the country. Now my parents and sister (who have already asked the person in question) are suggesting that her boyfriend comes too. Now I don't want their sinful relationship in my home, and I don't want my wife to be forced to house a non mahram non muslim man either.. My parents are trying to bully me into it as they invited my wife to stay here in the early days, and are accusing me of double standards. They are saying this boyfriend is not the usual short term fling and that they are serious about each other but I don't know him, and I don't like being bullied into compliance with something I am not happy to have going on in my house. I.e. I don't want them to sneak off into their appointed room for alone time, as I like my house to be clean and pure and don't want to be supporting their relationship which I don't see as legit.