First, lets be glad you got all this anger off your chest on here and not on someone else. Especially your H, and I'm very proud of you for keeping it separate. That would be very easy to bring old stuff back up.
second, you really do have to realize, as OT said, no one is perfect, and in this society, the society that almost glamorizes affairs, (see any actress/actor M history), it is so so easy to fall into the temptation of cheating. I know exactly how you feel Nik, and perhaps, and most likely that will never be you, (I say most likely because you can never say never, but you've been thru the pain on the other side and that will be a reminder to you if ever temptation passes your way). However, it's not that she has no morals, it's the fact that she still loves her husband but he was not meeting her needs and she had been suffering, not knowing how to communicate properly, and then in walks someone else who DOES meet her needs, and it consumes her.
If this had not happened to me 8 years ago, I would be thinking exactly like you. I WAS that person who would never in a million years cheat on someone. Everyone and their dog would never have thought I would do that. Thankfully I did not ML either, but it came really close, and looking back I am so shocked that I did that. It was like it was not me. I can't explain how it happens, but it's really like quicksand, if you just start to step in, it pulls you and pulls you and you can't get out.
I know that this IS still fresh, but try to release the anger because she is a sinner, like I am a sinner, and you are a sinner, and everyone on this planet. Some will sin huge sins, and others may be mother theresa. But we must be open to forgive, because she is hurting and she is lost and I hope that you will be able to share this place with your brother and that he will be open to learn from it. I know he is hurting, because it is the worst experience a person can go thru besides losing a child.
Right now, I would suggest to just validate your brothers feelings, do not say things against her... think of how you felt when your mom would say bad things about your H during your sitch. I know you were trying to save things then, but your brother may decide to try too in the near future.
but, on a good note, I'm very glad that you have realized the work load at home difference, and that you plan to talk about it constructively. Remember that Michelle said if someone is doing 90%, then the other person will only give 10%. And because your H is a guy, you most likely will need to spell it all out for him, with what you need him to do and help you with. I know, it would be nice for them to just get up and help, but they need specifics.
take care, and try not to transfer some of your pain from H, onto your SIL and father. I believe that is probably part of the problem too.
((Nik))
Me 33 H 34 S9 S3 M 6 yrs (2gether 11 yrs) EA/PA 1/2006 DB 5/2006 H wants D 6/2006 H wants ME 8/2006 H "said" PA/EA over 8/2006 H erased OW off phone! 2/2007
"It is far better 2 choose humility & change oneself, than 2 wait in vain trying 2 chang someone else."
But we must be open to forgive, because she is hurting and she is lost and I hope that you will be able to share this place with your brother and that he will be open to learn from it. I know he is hurting, because it is the worst experience a person can go thru besides losing a child. 
