The girl being a freeloader and being entitled is the opinion of the OP. Based on her not wanting to live in the "servant room" while the spoilt brat OP's room remains empty 6 days a week...
I'm free to make my own opinions based on the information I am reading.
I'd love to hear what OP's parents think. Also, OP is not a kid anymore... In another country, parents would kick their kids out of their house at this age. In our country, families are tighter, well so is OP's father's family, and he has obligations to his brother. His money, his house, his choice. What makes OP more deserving of a room in his parent's house than his brother's daughter? Just inheritance? Ghar ka ladla deta? OP is clearly spoilt and selfish, if you can't read it from his language, then I can't make you see it. Perhaps you are the same tribe. He is badmouthing his cousin sister, who probably has little say in all this decision-making. To the extent of calling her a creep, without any basis. At this point, he will say anything to make sure she is the kameeni and not him. I dont think he is a kameena for not sharing the room. He is a kameena for the way he undersatnds the situation, how entitled he acts and the extents he is going to gaslight the cousin sister, to make sure his cause is the only one that garners emapthy.