my bf and his sister acting like a couple

Hi. I have been very hesitant to post about this but I really need some advice. I (19) and my boyfriend (21) have been together for a year and a half. We have a lovely relationship and I really do love him. But I will get straight to the point. I am very close with my family. And he is very close with his family and has grown to be close with mine aswell. But no matter how hard I try I can’t seem to get close to his family. I always invite my boyfriend to my family events but he doesn’t do the same. For awhile I thought that maybe he just didn’t want me getting close to his family too fast but unfortunately i don’t think that’s the case. Him and his sister are weirdly close…. his sister is my age and i have tried to get along with her and go out of my way to be friendly whenever i see her but she is always cold to me. I recently was at a funeral with all of them because their grandmother passed and at the funeral I went to sit by her and she put her hand in the seat and said she was saving it for my boyfriend. They also went outside and took pictures together after the funeral and she was like all over him. Hand on his chest and laying her head on his chest. Just completely gross thing to do to ur sibling in my opinion. And the crazy thing is that their mom encourages it ! She prompts them to act like this. I need to know am i just being jealous?! I have never really struggled with jealousy of other people and my boyfriend but after witnessing this i just don’t know anymore.

Oh, last thing… i met the sister’s boyfriend and he pulled me aside while they were taking these photos and asked me if i found it odd. I told him i did. And he asked if i knew about them hanging out almost every evening. I was confused and asked him what he was talking abt. He said they’ve been hanging out completely alone in the evenings while I am at work. I had no idea. Why would my boyfriend keep that a secret from me ?! Please. Give me advice 😭🙏