It’s really common for children to retract a disclosure of abuse or to want everything to stay the same. Telling people makes it more real and means things change, which can leave the child feeling responsible for the fall out.
Right now you’re in shock, so will struggle to think clearly. You’re DD needs to know you believe her (which you’re clearly communicating to her), she also needs to know this is for the adults in the house to deal with - it’s not up to her to decide what happens now.
The Lucy Faithfull helpline is excellent - non-judgemental and very experienced, they’ll help you work out what you need to do here in terms of keeping your DD safe. As hard as it is, your DS will also need support because he too is still young - that doesn’t mean you need to give that support, but he will need someone. Again the Lucy Faithfull helpline can help with that.
I’d also contact social work - they should have services for young people showing sexually harmful behaviour and can make an assessment of risk and do some safety planning with you which may mean finding alternative accommodation for you son at least initially. Is there a relative or friend he could stay with at least initially?
If there isn’t somewhere else he can stay, you need to ensure they aren’t left unsupervised together - he may not try to harm her but may put pressure on to retract, blame her for telling or otherwise make her feel bad. Be clear to both that it’s a precautionary measure until you can seek advice.
Your DDs safety and wellbeing are your priority here, but so is your sons. Your DD needs support to understand she isn’t to blame here, even if she thinks she gave consent, because as a child she is/was too young to understand what he was asking. She also needs to know that your priority is keeping her safe, so living arrangements may need to change. Try not to treat her differently or keep referring back to what happened, give her space to talk when she needs to but also accept that will be on her terms.
Its very difficult and the natural temptation is to minimise and make it go away - you’re doing the right the tackling it head on. Do you have anyone in real life you can confide in?