I'm so sorry about what you're going through, FoL.
And I'm even sorrier for Si. It's been stated that she was never asked to help with the kids, she offered. Fair enough, but it's also been stated that she thought of the kids as her kids. Did anyone ever try to disabuse her of that notion? If not, then how can barring her from the house remotely be on the table?
Matt, how can you treat another person that way? Where is the empathy for someone who's given and supported and taken on a mother's place in her heart and was never told "this bond will be snatched away from you with no recourse if one of us suddenly decides, with no warning, that they don't like you any more?" Can you try to imagine being in that position? What has she done that's so awful? What has she done to make her such a disposable person to you, someone whose needs and feelings don't mean a damn when compared to your own?
I said it earlier and I'll say it again -- you have a perfect right to want to break up, to need space, to not like someone anymore (though I'm confused as to what about her has made you dislike her so deeply?). But I DON'T think you have the right to say "you are banished from my life entirely", especially when the person in question has done nothing intentional to hurt you (that I'm aware of). It's not just your life. Your wife and your kids share the space with you, and there are long-term bonds of love between them and this woman.
Why does none of that matter because you've come to find her annoying? How can you think of a human being who had loved and supported your family for so long as something to use, excise, and discard, how can you think of anyone that way?
It's enough to make me want to cry. Have a little compassion. Step out of your own shoes and into hers, for just a minute.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.