I feel Matt's anger at Si is misplaced. She took on the role you allowed her to take. YOU ignored Matt's opinions and needs for over a decade. Si just went along with what seemed to be OK with you.
We do see that sometimes here. The partner gets super angry at the metamour because they love their partner and can't bear to be angry at her/him.
Sharing time with her to be with the kids once in a while? You seem to have at least one nanny, who even spends overnights? So what if Si goes to your daughter's ballet or takes her out for a ice cream now and then? She could perhaps do it when Matt is at work, or you and he are away on one of your romantic getaways.
And again, it's all so moot. You, Matt and the kids are moving, what? 1000 miles away in a month or 2? Or is it 2000 miles?
Unless Si can manage to afford to take time and major $$$ to fly to Australia on a regular basis, her relationship with your daughter could die a gradual natural death. She won't have many ways to connect with the kid besides Skype.
Does Matt really "hate" Si? Is she that terrible of a person herself? Is she a danger to your kids? Or he is ragingly angry at you and your selfish ways? Angry at himself for not speaking up firmly enough for 12 years??
I don't really understand y'all's lifestyle. I live so simply. I never had a nanny, rarely could we afford babysitters, or meals out, or romantic getaways, when my kids were young. Our parents were hundreds of miles away so couldn't babysit. Now I see you spending hours at the gym, at the club in the middle of the night, somehow creating incredibly gourmet desserts with a 10 month old in the house, and throwing an "intimate" dinner party for 20! With a 4 year old and an infant, I'd've felt proud to get some spaghetti and a salad on the table. (And the toddler throwing noodles on the floor.) All while this stress between you and your partners is going on...
I just don't get any of this, I guess.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37