OP what a shock to get. I didn't want to read and run because this reminded me of an event from my childhood. Without overly sharing details, similar age gap, though both younger age profile at the time. No molestation, but inappropriate comment and touch. This has been something I've felt uncomfortable about as an adult because of how it was handled by my DP, and that really resurfaced when I had my own DC. I thought it might be helpful to you to share what I wish the response had been.
I wish that it was treated seriously. I don't mean punitively, just that it was acknowledged this is not ok. Dismissing it, not discussing it, or minimising in any way is harmful to a young child. Even if the event isn't something you yourself seem as severe, it can shape how your DD will view and place boundaries throughout her life. I wish this response was immediate. My wellbeing mattered more than their need to process or find the right time.
I wish that I had received therapy. Your DC may say they're fine, or have a close relationship with her brother still - it doesn't mean an impartial therapist wouldn't be beneficial. Regardless of the timeline. I am 3 decades on, and only just came to true acceptance. Therapy would have helped achieve this earlier.
I wish my family member had received therapy as well, to help them.
I wish my family member had been clearly told this behaviour was not ok. Regardless of the chance of reoffending being low (and it didn't happen), I wish as an adult measures had been put in place to prevent this being a possibility. Open door policy, no time alone, etc. I do not trust my DM with my DC as a result of them not implementing this.
I wish as an adult that we had had open conversations about consent and boundaries after this fact.
I also wish I knew what my family members motivation had been. I don't think it was sexual in nature, and often isn't with child on child occurrences. I worry that they were exposed to something they shouldn't have been.
I wish I could talk to my parents about this, but know I can't without it being seen as accusing my family member of being a paedo - something I truly don't think they are. I wish there was open discourse.
I hope this helps, but it is clear you care about both your children. This is heartbreakingly common, and you will be able to move forward as long as you don't minimise it.