Would it bother you if your younger sibling lived with you and asked to move their boyfriend/girlfriend in?

My partner aged 35, bought a 2 bed flat 3 years ago. Because he was single and didn't want to live alone asked his younger sister aged 19 to move in with him. She started dating someone shortly after and asked if he could stay over. Eventually he moved in. That relationship broke down after a year and she started dating someone else. The same thing happened again.

Context: my partner says he views his sister like his daughter because there is 11 years between them and he looked after her a lot when his mum returned to work after 3 months of giving birth. His sister doesn't pay rent because she claims she and her boyfriend don't earn enough and he pays all the bills, buys food shopping for the household and also cooks them dinner. The only responsibility they have is keeping their room tidy.
Would it bother you if your younger sibling lived with you and asked to move their boyfriend/girlfriend in?
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