Boyfriend and his stepsister

Ok so first off, I guess I am kind of young. I’m 17 and so is my boyfriend. He told me that he cuddles with his step sister a lot. Like he lays on top of her and plays with her belly button ring while she rubs his head and back. I’m not sure if I’m being over dramatic or what, but to me it’s weird and I don’t like it at all... I myself have a brother, and I literally have never cuddled him since he was like in diapers. Him and his stepsister are obviously not blood related, but they are related because he fell into her parents custody when he was young. He has a younger sister and brother and other step siblings, but he literally chooses to cuddle with her. He also has her contact name as “My Queen❤️” in his phone. He says it’s like that because she has depression issues and doesn’t feel wanted, so he put that as her contact name on Facebook messenger so she will feel important and wanted... is this something I should be worried about? Or am I just being over dramatic or too analytic over the situation? I haven’t told him how I felt about it at all cause I don’t want to sound like a bitch at all or anything.