to search for a biological sister or not?

A bit of background info before I ask my question: Several years ago, my husband's grandmother passed away. When he went to clear out her apartment, he found a hand-written letter, addressed to his grandmother. Reading that letter, he discovered that his mom had given birth to a daughter a few years before he was born, and she gave that baby up for adoption. He had a biological sister he'd known nothing about. His mother had passed, his grandmother had passed, his aunt agreed to fill in what blanks she could, so he put them in touch. His sister simply wanted to know some medical information and have some questions answered. She did not want to remain in contact - no reunion. He asked me, as an adoptee, to tell him what to do? I told him he should follow her wishes. So he hasn't been in touch with her since that first discovery. Last year, I searched for and found my own biological family. Getting to know them, being welcomed into their family, enjoying them and seeing them enjoy our young son... Now my husband doesn't have any real family to speak of, so I think it's been a bit bittersweet for him, feeling like he doesn't have any family to offer us. I guess it's been making my husband (and me, to be honest) wonder if maybe his biological sister might change her mind... So here's what I'd appreciate some thoughts on: Should he attempt, one time, to re-find his biological sister? Should he reach out and make sure she knows the door remains open to contact? Should he let her know she has an adorable nephew who'd love to have more family to love? Or should he stick with my initial advice, which was to back off and let the adoptee decide?