Quick as I can:
MOG and FI have had an on again off again relationship since he was 10.  Had a new bf- sent FI to live with her mom, when the relationship fizzled he'd be back with her. 
He made a mistake last year, and now it's my fault.  (i preface that there is no violence in our household) but we got in an intoxicated fight about something as stupid as i was smoking in the basement and didn't ask him if he wanted to smoke too.  He stormed out and got in trouble.
I carried a big load and a few months back went to her house to help her, we were talking wedding, life etc. and i asked her how a mother of 2 (currently in the house has a 6 & 9 year old siblings- he's 30) how she did it all as I was tired feeling like i was carrying the load these days.... It turned out i was in the wrong and i was not only blamed for upsetting him that night months prior, but i was not a role model for his daughter for not being strong enough and if something happened to him i'd let him waste away.  I stood my ground clarified my statement and said he's 30 i do not need to take the blame for his mistakes.
His cousin got married 2 months after that, i changed my mind about the dress I was going to wear (pink) because i got these fab shoes that didn't match so i wore a black number, she wore something similar and only words said to me was i thought you were going to wear pink.  She proceeded to try and get him to be next to her every second of that wedding and dance, one dance he said, "mom i'm not going to dance to baby got back with you"
Fast Forward, she's not coming to the wedding, I'm a horrible person etc etc. (mind you the relationship has been on and off for 20 years) and everything under the sun, he asked me to apologize in july, i refused to apologize for something that i am not responsible for, he was hurt but understood.  He talked to her just the other day and says she still can't get over the fact of what happened in May and how threw him under a bus.
I told him that he deserves family at the wedding, he doesn't have  close relationship to his dad at all. I told him that i should've wrote the letter back in july to apologize (then i could phrase it so i wouldn't be lying nor give it away i wasn't sincere- my face tells all)  he said if it wasn't this incident it would be something else and i'm the scapegoat in this, that before when she was mad at him she'd call me to get info and now she's blaming me and if it wasn't me it would be something and asked me not to write it.
I feel bad, if it was my family..... i'd be devastated but he's dealt with this for a long time.  Should I write her a letter? do we invite her and her kids and husband? do i just let it go?  i feel badly i think mostly because if it was on the other foot that my folks were shunning me because of him.....
thoughts? i just feel really bad and our talk last night about it all ended that i wouldnt' do a thing... he's been calling her by her first name for months as she's no mother to him...... but when his dad's grandparents die, he really will have no family connection and i want to fix it
edit- ran thru spell check; i've re-read it i don't think punctuation etc is really deviating form the story. 
Nutshell she's blaming me for an error he did, I didn't take responsibility, she's not coming to the wedding because i didn't apologize, he doesn't want me to because she's always finding a reason to push him away and now it's my fault.  I feel badly because if i sucked it up and apologized months ago we might not be here now with her BS now.  But i stood my ground previously and didn't, and he doesn't want me to now.