Just curious....

My brother's ex and he have a good co-parenting relationship. My brothers ex is currently 6 months pregnant with her new mans baby and my brother has been happily married to his wife for 5 years. His ex and his wife have a decent relationship. My brothers ex always ask their child if my brother and his wife are happy, do they show PDA when the child visits and she's constantly asking the child about what her father is doing and what is going on in his life. I'm confused because I thought if she was with someone else and currently pregnant by another man, then what my brother and his wife do is none of her business as long as the child that she shares with my brother is loved and well taken care of. ???