Sister kicked me nephew out Update

My nephew just came to my house and it's 1 in the morning. It's his 18th birthday. My sister literally woke him up in the middle of the night and told him he's 18 and needs to go. Before anyone says anything there aren't any behavior issues or anything. She had always said when her kids turned 18 they had to get out. But I don't think anyone expected her to wake them up in the middle of the night on their birthday to kick them out. I called her about it and she was mad at me because SHE was trying to go back to bed. She said he's 18 now and her legal obligations are over and she has two more kids to get through. He's still in highschool and I have 3 kids myself. At the moment I'm letting him sleep on my couch. I just don't know how to help him.

Update: So I told my nephew if he can't find anywhere else to go he can stay here. He will have to sleep on the couch but he can focus on finishing highschool and we will figure the rest out as we go. I'm gonna try to get him a cake although I doubt it will save his birthday.