My flatmate is living with his sister.

I live with 2 other guys in a 3bhk flat . So one of my flatmate said to us that his sister is finding a new flat (she was also living in same city) so she'll be living with him for 1 2 weeks. Initially this was fine. Cut to this day, it's been 1 year since she is staying at our flat, with her brother. Although, she stays in her brother's room (they both live in same room) but I'm a bit uncomfortable in this setup. On top of this, electricity, gas, wifi bill were divided in 3. Now finally I'm planning to confront him about the same, but I'm confused, should I or not. Also if I confront him, what should I say to make sure things go smooth, and he understands it. My another flatmate, don't give a shit about all these things, so won't get his support on this. Asked my parents regarding the same, they suggested me to adjust 🥲.