OP I've just read page 2 again.
Future children - Christenings, birthday parties, Christmas, school plays, celebration assemblies, graduations, your children's weddings...
If you don't nip this in the bud it will never go away.
I have a cautionary tale:
Until my son started school we hosted 2 birthday parties for him - one to which ILs and my dad and his wife were invited, another to which my mother, brother and grandma were invited. My brother and grandma would have happily come to the 'main' one, but were emotionally blackmailed by my mother into attended the second one with her.
We held 2 separate Christmas parties until my DS was 7 for the same reason.
My mother completely expected this to happen, felt we were disloyal if we invited my dad's wife to anything and made a big deal of showing my dad (oddly by flirting with him) how 'over him' she was. They divorced 20 years ago too.
My mother managed to ruin my son's 6th birthday party. It was the first time we'd done something to which we'd invited everyone. I told my mother before issuing the invites that we'd be inviting dad and his wife. Her reply? "well of course you would. It's been a long time. I'm sure we can all behave like adults for the sake of X's birthday".
Great! So I issued the invitations and when dad and his wife accepted, I told my mother straight away (so she'd have time to get used to the idea). And again she was "Oh is she coming? I'm surprised about that, I didn't think she would." etc etc.
The day before the birthday party my brother phoned up in a foul temper telling me that he was going to call my dad and uninvite his wife to my son's birthday party. I told him he wasn't going to do that. Cue - huge row between my brother and me. My grandma phoned and told me she thought I was being incredibly insensitive and selfish to just drop this on my mother and did I have any idea how much of a shock this had come to her.
Obviously, just my mother playing the victim again given that she knew I was going to invite dad and his wife before the party had even been booked and no one else had been invited.
Anyway, it caused a huge hoo-ha. Completely ruined my experience of my son's birthday party, made the whole thing incredibly stressful and all because my mother was such a selfish, inconsiderate, self centred bitch that she'd got not only my husband and me running around trying to keep her happy, but that when that wasn't enough for her and she wasn't the 'victim' she dragged my brother and my grandma in too.
Sorry this was long and sorry for hijacking, but you really do need to nip this in the blood.
Since my son's birth (tbh it was easy for them to avoid each other before that) there has been my graduation; my brother's graduation; the birth of my second child; the birth of my brother's child; several birthdays (mine, my brother's, my SIL's, my husband's, my children's) and every Christmas. My engagement (a whole other confusing and, frankly, unbelievable story), my wedding, my brother's wedding... Nativity plays, drama club performances, music recitals... the list goes on) and every single one of them has been blighted by my mother - or over the top consideration for her feelings.
In every single one of them has she been the only person who has received personal consideration and for every single one of them (without fail) she has kicked off/made a fuss/had a tantrum.
I'm 38 now and the last 20 years of my life have been largely controlled by my parents divorce. I should have put a stop to it years ago. In fact, I should never have allowed it to start.
Learn from my mistakes! 