Am I overreacting about my boyfriend telling me something about my sister?

Am I overreacting about my boyfriend telling me something about my sister and her boyfriend? My boyfriend’s coworker is friends with my sister’s boyfriend. For some back story my my sister and her bf have had a very rough relationship and they weren’t seeing each other for the last 6 months but they got back together without telling anybody. Basically the coworker told my boyfriend that my sister was caught with her bf doing -stuff- loudly in his parents house. Now I really don’t care what my sister does but it’s the fact the my boyfriend was telling me this. He already knows all the family drama with my sister and i gossip about her (like siblings do) with him, but I just feel like this was one thing I really didn’t need to know especially imagining that my boyfriend is picturing my sister and her bf in bed together. like that’s just gross. I’m also just feeling weirded out cause I’ve noticed how my sister acts infront of my boyfriend, and like flaunts herself or puts extra effort in her appearances when she knows he will be around. I guess this is more of a vent now. I want to clarify I trust my boyfriend and I think he was just gossiping with no intention behind it but it just feels strange knowing this and I guess it brings up feelings of insecurity knowing how my sister is. Idk it’s just weird. Am I overreacting and does this even make any sense😭

• I did not flip out or get mad at my boyfriend or anything it just came as a shock hearing that coming from him