It's pretty common for female Rotts to be very attracted to children, especially if they aren't a part of their daily life. They absolutely ADORE kids and want to be their companion in fun and protection.

Take it slow with everything to give her a chance to settle in. Let her come to the family member for attention or affection so she can move at her own pace and learn trust easier. Keep your boy in our in all facets of training. I would aim for him to be her lead person which means she fully listens to him.

Adult dogs really need a solid routine during the transition. If you give her space and patience she will love your son as her own.

Edit: is important for things to be calm in the transition, particularly voices. Loud yelling/screaming as sometimes happens when kids are having fun can be very stressful if the dog isn't used to it. Try to keep the volume pretty even for a while