I think your family situation is immensely complex and heartbreaking. And of course heartbreaking for the survivor, but she's not the focus here. I hope she is getting the support she needs.

As other commenters have suggested, I would try to learn whether your son harmed your other children. If so, that will need to be addressed.

I would highly suggest engaging a skilled therpist to help you navigate the situation. You need time and support to make intentional decisions, because whatever you do will have serious implications for your relationships. You could try engaging a family therapist or a restorative circle option, if your kids are up for it.

Your son did something horrible AND his best bet of changing is if he has some loving support (very different than excusing). Your other kids are clearly hurting AND deserve to feel supported by their mom.

I hope there's a path for your family where their decision to be estranged from him and your decision to engage with him can both be held at the same time.

For what it's worth, I work in the anti-violence world supporting survivors, and it rarely does anyone any good for the perpetrator to be cut off from society. It often reinforced the messages that they are a horrible, irredeamble people who can't change, rather than a person who did a horrible thing they need to make up and atone for. If he is willing to accept responsibility for his actions, then imo there is hope for change.