Help me clarify my feelings about family issue

[showtime7]

I will try to keep this short; I guess I am writing to see what others would feel/think/do in this situation. Basically my older sister is behaving in a shitty way and its been going on so long that its getting ridiculous. She left her husband of 20 years five years ago. Her two boys were grown up. She then met someone I will call Man A (its confusing). I knew Man A from work years ago and his wife had died a few months before my sister met him. She started seeing Man A although he stipulated from the beginning that he didn't want anything serious. Man A is quite a closed book and he does not even particularly mention anything about my sister to me when I very rarely bump into him. To be honest I think my sister always felt more for Man A than he did for her. Anyway, a few months later she started seeing Man B who is quite a close friend of the family. They moved in together. Four years later she is still with Man B living in a rented property together. He is considering buying a house with her. The problem is that for the whole time she has also been seeing Man A on an intermittent basis. Man A has no idea about Man B. Man B I think has had some idea that my sister has a couple of male friends and possibly hasn't always been entirely innocent in their relationship but they have always discussed this and she maintains her 'innocence'. Man B certainly has no idea of the extent of her cheating. My sister often says to me that she will stop seeing Man A but can never seem to stick to it. My sister's sons have kind of guessed whats going on but try to keep out of it. Our brother and his wife know about it; my brother gets quite angry about it all but his wife says that we should not get involved at all.

I very much do not want to get involved and keep out of it all as much as I possibly can. But it is difficult when you regularly see the different people involved (my daughter is quite close to her cousins, etc). For a long time I supported her and tried to help her make the right decision but now I feel it is difficult to keep respecting someone who is behaving like that. Please help me sort out my feelings about this. How would you all deal with it?

[mrstweefromtweesville]

Nose out, that's the line I'd take.

[QuiteLikely5]

I don't think it's something you can do anything about. Realistically what can you do?

Her behaviour is appalling but it's her problem to solve.

Do nothing.

[SpanglesGalloway]

To be honest she's an adult and I'd leave her to it. Unless she asked me for my input...which it doesn't sound like she has...id leave her to get on with it. There all adults

[showtime7]

Actually I suppose what I meant is how do I deal with my own opinion about that. I agree, the behaviour is appalling. We used to be quite close but it's like she's a different person who does not give a toss. Either that or she truely thinks it's ok to treat people like that when we certainly weren't bought up that way. Our dad doesn't know but then I think if he ever asked how could I lie? He would be absolutely mortified. I hate the way it makes me feel.

[cheapskatemum]

Have you told her how you feel about the way she's behaving?

[showtime7]

I have and then she says it's all going to stop. But after that she will let slip that it's all still going on. I am one of those people that can be completely understanding etc but it's getting to the point where I feel that if I did say something now I would let rip and that would be that. I guess I have to just keep distancing myself.

[Aussiebean]

I would also make it clear that you will not lie for her. IF someone asks you what is going on, you will tell them.

After that, do not discuss it with her at all.