Entitled girlfriend demands sister-in-law let her use her house to have a girls’ night, doesn’t even ask for her permission, and gets offended when the house isn’t available.

If you rent a house, that is your house, period. 

Some people don't understand how the concept of "rent" actually works. When you pay the owner of a house to live there, as long as you do it, you can do whatever you want with it, including letting people in or out. It doesn't matter if the legal owners of the house are your parents, your neighbors, or the king of France. 

A daughter was renting her parents' house for $600 a month. They got another house, and she moved into the old one to keep everything afloat, paying a modest amount of rent and taking care of the property. Meanwhile, her brother lives somewhere else with his girlfriend and their 3 children. One weekend, the girlfriend asked her mother-in-law if she could use the house for a girls' night, but the mother-in-law answered that the house wasn't empty, since her daughter lives there and her boyfriend was coming to visit, and that ultimately she should ask her daughter if they could use the house. The day came, and the boyfriend's flight got delayed, so the daughter went to her parents' house to help them babysit her nephews. He brother called sometime later, complaining that the house was empty, that they could have received her friends there and whatnot. Funny thing, they hadn't even asked her if they could use the house. The brother and girlfriend made such a fuss over it, apparently considering that it wasn't her house, that she wasn't paying real rent, and that they could use it too, that the parents ended up increasing their daughter's rent from $600 to $1500, all to make the brother happy.

In situations like this, we would most likely want to tell the parents to grow a spine. Their son is a fully independent man, with a girlfriend, a house, and three children. He doesn't need to worry about his sister's living arrangements, especially when she takes care of their childhood home without any help. She's not living there for free; she's paying a reasonable amount in rent and keeping the property in good condition. The parents' decision to increase that amount just because their son complained is not fair at all and shows a tremendous lack of authority on their part. If the previous arrangement worked before, and they were all happy with it, there was no actual reason to change it.