I am so sorry you are facing this. I just want to offer a slightly different perspective to some of the responses you are receiving. I say this as a survivor of childhood SA myself.
Your husband was 15/16 at the time - legally still a child, and developmentally still very much an adolescent. He was old enough to know right from wrong, but at that age, understanding of consent, boundaries, and consequences is often immature, and can be further shaped (or distorted) by factors such as family dynamics, trauma, and the environment he grew up in.
What he did to his sister was profoundly wrong. It was coercive, abusive, and harmful. His sister’s feelings today are absolutely valid and must be respected fully. She owes him nothing, and her boundaries must be honoured.
However, there is a meaningful difference between child-on-child abuse (particularly when the age gap is relatively small) and adult-on-child abuse. The harm is real, but the context, the development stage of the perpetrator, and the longer-term consequences can be different - for both victim and perpetrator.
Recognising this does not excuse what happened, but it can influence how you think about accountability, change, and healing now.
It's important your husband has taken full responsibility, apologised, and is not trying to deny or minimise what he did. That matters because it shows capacity for reflection, remorse, and moral growth.
I encourage you to seek support from a specialist organisation that understands sibling sexual abuse and can help you work through the complexity of this. Specialist counselling could be really helpful to you, whichever path you eventually take.
Sending you strength OP as you work through this.