I hope it's sarcasm when you say that her needs (presumably to stay involved with the children to whom she's acted as a mother... pretty dang relateable and reasonable and human) are as reasonable as his (her complete banishment from your "lane" of parenthood, with no acknowledgement of the fact that it's de facto been her lane too for five years, for reasons that boils down to "I resent it and that matters more than your pain, her pain, or the children's pain, and compromise essentially doesn't exist to my mind.")?
I found myself thinking just now about what "family" means. Your daughter has been taught to see Si as family. And not extended family, like an aunt, but nuclear family, a mother, part of the core unit of her world. What lesson will it teach her to find out that a family member can be banished, that designation as core, indispensable family lost forever, when someone decides they don't like you any more, even if you didn't actually hurt anyone? Will she grow up with the fear deep in her heart that, if she makes someone angry, she'll suddenly stop being family too? Or that she'll suddenly and forever lose mom #1 the way she lost mom #2, for reasons she doesn't understand?
Even in a divorce where you have to have the hard conversation to explain that Mommy and Daddy don't love each any more and that's why they live apart, there is always the strong emphasis that the child is still loved and gets to keep a relationship with both. Because it's not the child's fault, and it's not okay to break that bond just because the adults are having problems amongst themselves. Even in a divorce with a harsh custody battle, everyone understands that visitation rights are inviolable unless there is a reason to suspect there would be abuse or neglect.