Re. outside support: My H seems to be irritated and uncomfortable whenever he thinks I've told someone about what's going on--he is very concerned with his reputation with friends and family. I found that the more I talk to certain friends, the better I feel. I think women are often wired this way, whereas men seem to keep stuff to themselves. My friends are my main support aside from this board. H may not like that I confide in them, but I need to to stay sane, so too bad. When he chooses to turn back to our M, then we'll talk.
Anyway, I did tell my family, but I don't share many details with them, especially my parents. When this is over, I hope H is still part of the family, and too much info may be a bad thing then.
I hope that you can share this burden with your sister and that she can ease the load for you a bit.
