Be honest with your daughter. Explain that her dad and Si are mad at each other and while they both love her, they don't want to be around each other.
I've been in similar shoes to Matt in that while I was working and going to school to support us and get us further ahead, Runic Wolf's first poly relationship was developing. She semi moved in, started cleaning my house, cooking for my family. None of that bothered me until I got an unexpected night off and they'd planned to take my son to the carnival. I was excited to go too and she told me I wasn't invited. The next day at pre-school, my son drew a family picture with her in it instead of me and I flipped shit, told him she had to go, and that was that. There were other things going on at the time that took a while to sort through, but that was my limit. And still, I wouldn't advise you cutting her from your children's lives. My son doesn't remember my husband's ex, which is for the better after the emotional abuse she put my husband through that I'm only now learning about, but my son has an extended network of aunts and uncles who are close family friends. Some are friends who've been roommates for short periods of time, some have been our friends since before he was born, others since he was very young, but not once did I regret allowing him to build these relationships.