Needing advice for weird situation with roommates
So i currently live in a house with 3 roommates, originally it was myself, a friend of mine, and two of his friends, lets call them S and T, from his last living situation. S and T are dating, and S's dad rented us the place. My friend has moved out already and was replaced by T's brothers girlfriend, Z. Since Z has moved in and officially put on the lease, Z's boyfriend and T's brother C has been living here as well. I sent a message to our groupchat asking if C is living here as well, where I was told he was only there to help clean, and would be out the next day. At that point C had been living there for a week, and our lease says guests are allowed for periods not lasting more than one week. Its now been 2 weeks since then of him consistently living there, and the roommates left a non-landlord approved message taped to my door saying I will need to be moved out by the renewal of the lease. I am not too keen on paying increased utilities and paying my regular share of rent while C stays here for free. I plan to bring this up to the landlord come next week with that being 3 full weeks of C being there, but I dont know how to go about that properly with his son and his girlfriend being the root of the problem. I had a friend also message me asking how it is living with T, S, and C, so i think they are being purposefully sneaky with me about the reality of the situation, which is most likely that he will be replacing me come the renewal of the lease in a month. How should i bring up the fact that C is living here unoffically to the landlord? Thank you for reading, I appreciate it.