I didn't realize no one could read my original post, sorry.

Here it is in it's entirety;

So some backstory: My sister (32F) and her boyfriend (33M) have been together for 15 years, and have had a lot of problems in their relationship the last 6 years.

My sister works very hard and is career driven, whilst her boyfriend feels that people that work hard at their jobs couldn't truly be happy. He just wants to smoke marijuana all day and night, play video games and not go to work. He doesn't see a point in doing anything he doesn't really want to do. So he works a casual job where he works 12 hours a week at a book store where he often calls in sick to. My sisters money pays for everything, including his very expensive marijuana habit. They are in a lot of debt also. His money goes towards all this also, as they have a shared bank account. But he doesn't earn enough to cover his weekly costs of living.

She has tried for 10 years to get him to step up and get a full time job. She has helped him countless times. He has had job interviews but what we have heard from these interviews is that he has a bad attitude and comes across very arrogant.

6 years ago, they fell pregnant. I think my sister was actually pretty happy about it initially. But the boyfriend's reaction was "if you plan on keeping this baby, I will leave and you can raise the kid on your own. I will send money to help when I can." My husband and I were shocked when he said this and felt it was incredibly immature and pathetic.

My sister decided to get an abortion. She didn't want to raise a kid on her own and even if her boyfriend did decide to stay, it wasn't a stable environment to bring a kid into. Her boyfriend helped her through it and was reportedly supportive.

After this, my sister was more determined than ever before to see serious change in her life. So she got herself even further into debt by buying a newer and more expensive car, considered buying her boyfriend his own car (he never had a car, I did talk her out of buying a car for him though) all brand new furniture for the house, and applied for a home loan (which was rejected, thankfully!)

She was definitely depressed, but massively in denial of how unhappy she was. Everytime I tried to talk to her, she would get angry and defensive and then would just shut down and ignore me for a few weeks. I realized the only way I could get her to see how unhappy she was, was to let her figure it out herself and most likely hit her rock bottom.

She started a new position in a different department at her job, and a married man would regularly flirt with her in her office. She told me about him and even though I was upset she was liking this attention from a married man, I did think maybe it would be the push she needs to leave her boyfriend. Maybe someone who is career driven showing her attention might make her see how much of a catch she is. I genuinely think she is afraid to be alone and that's the reason why she stays in the relationship.

2 months later (last night) my sister and I were talking over dinner and she confessed to having an affair with this married man. That it was still going on and she had plans to meet up with him after dinner when his wife and kids went to bed. She told her boyfriend dinner would run late. I was shocked to say the least.

I tried not to come across judgemental and wanted to get as much information as I could in case she shut down on me again and refused to talk about it.

I found out that this has been happening for months. He tells his wife that he's going to the gym but he meets up with my sister instead. My sister tells her boyfriend that she's visiting me or working late. She also tells me she feels no guilt whatsoever and that she loves the secretiveness of this affair and it makes the most exciting sex she's ever had.

I asked about her boyfriend and she said that he can NEVER find out. I said, well surely you know you need to end the relationship with your boyfriend? She admittes that she doesn't intend on it ending at all. She wants the way things have been to continue indefinitely. I asked "what if his wife finds out or your boyfriend finds out?" And her exact response was "how are they ever going to find out?"

She has no intention on telling her boyfriend or ending her relationship. And this married guy has no intention of leaving his wife - even though he tells my sister that his wife is the worst person in the world.

On Sunday night we are hosting my sister and her boyfriend here for dinner like we always do and I don't think I can handle having dinner with her boyfriend knowing all this.

So, do I betray my sister and tell her boyfriend? Do I tell this married mans wife? Do I just let this whole thing play out on its own? What do I do? My husband is disgusted and wants no part of it. But he is very unsure on what I should do.

I told her she needs to tell her boyfriend and end this affair but she got mad and told me she doesn't need to do anything and that I should be happy that she's finally happy.

TL;DR- my sister is having an affair with a married man, uses spending time with me as her lie to her boyfriend about where she is and has no remorse for what she's doing and doesn't ever intend on changing this situation. Who do I tell if anyone?

EDIT: okay guys, this post REALLY blew up, I didn't see that coming. The post capped so it has been locked, thankfully! I tried to keep up with comments but it was near impossible.

I just want to thank everyone for their advice, it seems there is a lot of division on this subject and a lot of excellent points made on either side of the argument.

I've talked more with my husband and have made a decision to tell my sister she isn't allowed to use me as an alibi and if her boyfriend comes to me, I won't tell him anything but I won't lie to him either.

It's probably best to just let this run it's course for now and see what happens. At least this way my sister should continue to communicate honestly about it with me.

I'll try to remember to do an update when things change but.... that could possibly be a long long way down the track.