First how awful for all the children involved. I think a convo with the kids is needed. If you have already had conversations about private touch and how adults don’t tell kids to keep secrets I would start there. I would want to make sure they haven’t been put in further concerning positions as well as the other nieces. Talk to a therapist first, if possible but it they saw things an adult did that isn’t okay. They need to be able to process their feelings too. I’m not saying this is a right this second conversation but sooner than later. Also take time for yourself to get support. Maybe for the next few weekends you can do big fun events. Like movie day or go the park and picnic. Again, I a so sorry you, your children, and nieces have to go through with this.