Sometimes I think this happens because we are meant to give the sibling a new lease on life. If this had happened in their old home, Rosie would likely have had a horrible time. Instead she is with you and has new siblings and new experiences; letting her brother go will is easier than it would have been.
I adopted two parrots (live in separate cages, but next to each other) who had been together for 28 years. Six months later the female died of cancer. The male is now Dog-bird and is so happy. He goes camping and loves it. He is no longer overshadowed by his outgoing, friendly sister - but is now the center of attention. He has a new flock (the dogs). He will even eat other than his seeds when camping (other food is still poison at home, but when camping it is somehow OK). Even though her passing was sad and I felt like I had failed somehow, I now realize that I was meant to give HIM a new life. Andat that I have succeeded.