Him and his little sister talk about sex?

I been dating this guy for about 3 weeks, he’s super sweet and a great guy. Today I was over his house when his little sister called him. His sister is 17 he is 21. They were having a normal conversation until I hear them talking about sex. (I wasn’t listening in, he was right next to me) They were really getting into detail when talking about her sex with her boyfriend. Going into deep details like him cumming in her, how fast he cums how good it was etc. They’re just having this whole conversation about this teen girl and her boyfriends sex. Now... I understand they are close, but it just kinda grossed me out. i can see telling your brother about losing your virginity etc but the DETAILS ..She has sisters too, and friends, so to choose to discuss this with her brother I don’t really understand.. Me and my brother are close as hell, he’s my best friend and we still don’t discuss sex and if it’s mentioned it definitely doesn’t go into detail... My question to you is, do you find this a bit odd? Thanks for your opinions.