Husband sending explicit messages to friends sister

I was recently showed proof that my friend’s husband has been receiving explicit images from my friend Sister and also reciprocating.

So for context, my friend lives with her husband and her sister who has a child in a small house. They have lived this way for about 6 to 7 years. they are all very close and as far as I know my friend is really happy with the living arrangement and that her sister is so close and that they get to spend time together. I have been debating since I was shown this information whether I should tell her or not as much as I don’t like her husband my friend is very happy in her marriage and I feel like the information would really harm her.

Alongside being sent screenshots of the images and messages. I have also been in the room when her husband was with his friend and he stated multiple times that he likes her sister in a non-platonic way. I also worry that if I tell her she may struggle mentally she is currently in therapy as her husband suggested that she go due to her temper but I have seen her trouble for awhile.

Her husband regularly tells her what she can and can’t do what she can and can’t wear and who she can or not speak to. There have been regular instances where she has been told she is not allowed to see or speak to me due to me being a bad influence and causing her to make decisions that he does not agree with. I respect these periods and normally we catch up after a few months have past and he calms down. She said she likes talking to me as she can actually speak freely and not have to worry about him getting upset.

She also always shares that she worries what would happen if he wasn’t there, and that her life would be over if he decided to leave her,

I do wonder if anything has actually happened between the husband and the sister. They regularly share a bed due to my friend having sleeping issues and lack of space. my friend says that she does not worry about her sister being so close to her husband as she knows that nothing will happen but can I sit by and let this carry on when potentially they could be using it as an opportunity for him to cheat on her I really care about my friend and do not want to break up their marriage that’s why I haven’t told her anything that I know but I kept the information.

Do I tell her or not?

Edit: Apologies for the delay l've never posted before and I'm at work

To answer a few questions, the sister is in her 20s, 1 found out from Husbands friend as we work together. The husband posted screenshots of her pictures and his reply's in a group chat and the guy who told me didn't agree. Husbands face was not in pictures but you can see the typed responses. After reading the responses I am going v to tell her but make sure she has a good support network so she can have some help in case she does need to get out or use it, thank you for your advice and opinions