Brother & sister ??

Me and my SO have been together for 2 years we don’t really go to family parties but we did. Everything was going fine we went outside to talk. We were all in a circle my SO sister was sitting on the back of a truck and she asked my SO to sit next to her I didn’t think nothing about it. Until she pulls a chair and puts it between her legs and he sat there for more then an hour and she was hugging him like umm okay ? I didn’t say anything. Another party came it was indoors and they sat next to each other again she had her arm wrapped around him all night too he didn’t even bother to pay attention to me ? Is this okay behavior between siblings or is there red flags?? 🤷‍♀️