Weird sibling behavior? Or am I overthinking? Please give feedback?

A0210
I have a boyfriend that has a sister. We’ve been together for 3 years and nothing weird has ever happened between them until now. I started noticing he’s touching her butt and thighs. He has caressed her hips in front of me and I acted like I wasn’t looking to not make it weird. She calls him sexy. And she sits on his lap. I don't know about their sibling behaviors but I find it very uncomfortable and weird for me. I’ve spoke to my boyfriend about this and nothing can seem to be resolved. I’m not sure if that’s normal but the butt touching and grabbing and caressing does NOT sit well with me. Am I overthinking / overreacting or is this “normal” ?
Updates
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In the beginning of our relationship they have NEVER done this but now since we all live in the same house things are starting to change and those things started happening. I tried to explain to him how it makes me feel uncomfortable and that he needs to set boundaries with her. We are all grown now and it shouldn’t be normalized period. She has a fiancé and 2 kids. It’s not normal for me but to his whole family it is.
Weird sibling behavior? Or am I overthinking? Please give feedback?
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