Brother and sister relationship

My boyfriend of a year has a odd relationship with his sister (to me). I’m not sure if I’m just jealous or crazy. So they get undressed in front each other to change or anything. Like boobs out and balls out. They also sleep in the same bed with each other when there is space at his family’s house to sleep anywhere else. He also sleep just in underwear with her. I have watched him run his hand on her thigh up and down. Now she doesn’t like our relationship because she feels that I am going to take him away from her. Yes I have talked to her and so has he. Am I crazy?