"I saw my SIL sitting on my husband’s lap"

Her story: I found my husband and his sister sitting in a very weird way. She is a 20 year old college girl and my husband is 28. She was sitting on his lap and they were watching Basic Instinct together. When I asked them what's happening, neither of them moved, it was so natural for them... I have never felt so uncomfortable. I don't know how to bring this up with my in-laws. When I tried talking to my husband about it, he got offended. How can I not bring it up? They are adults! What should I do?

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