I need to vent

Anyways, my ex bf lives with me and when we were together he got reconnected with his sisters, well one I met in person, other one no. Other one I know is his sister they have same dad. And they were just adopted at young age. Well he was able to go back to his mom. Well fast forward abit, his sister #2 is married and kids same as #1.

#2 messaging him some times and asked some odd questions about or sex life, and just asking if he makes me cum alot and everything under the sun, #2 last night messaged again same stuff this time she asked him. He has to work in morning, she has a husband mind you. And she messaged him saying I'm touching my nipples and clit, do you have to go can you help me finish, omg please! . Well since last night got me thinking how do I approach this to him, I tried to tell him last night my concerns on it, I am still worried since he has cheated in past, but THIS IS HIS SISTER!!

And he says he ignores it, I don't know what to tell him, he's my ex yes we still have sex. But this is getting weird. Any advice