My sister Boyfriend made a joke about rape, I don't know what to do?

He made the joke in front of her and she just lightly punched him on the harm while I was just too shocked to react so I just turned my head and stared at the tv. I don"t know what to about this whole situation. If I say anything they would just say that I overeact as anyway my sister told me her boyfriend hate me and often call me a slut / whore.

My sister Boyfriend made a joke about rape, I don't know what to do?
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