My husband and his twin sister were adopted together. She grew up always wanting to know her biological origins, he did not care. After years of searching, she finally found them and a few months after meeting them for the first time, she packed up and moved halfway across the country to live with them. They keep trying to get us to go visit and meet the whole family, but my husband has not really shown an interest in it.
Your husband seems to be content living in the present. His sister remains focused on the past, on what seems to be missing from her life. I’m glad it worked out for her. Some couples make better parents of a young adult, and lousy parents of a helpless infant. That’s just who they are.