I never got the impression him and the sister actually liked each other. It just seemed like a marriage of convinience. He's a noble that can help her brother and get them out of poverty, while she's a younger good looking woman. The most basic hetero relationship imaginable. So of course he wouldnt have really known or cared about her, otherwise why would he let her have a kid when her health was at risk or why would he doubt her just because of some rumors.
I thought that as well but then he showed flashbacks that seemed like they were really close. Maybe his flashbacks are influenced by his own thoughts. Regardless, what I’m questioning is the logistics.
The side note: his brother in law suddenly becoming doubtful of his relationship with his sister like both of them were not both fighting the same war and having no time is crazy to me. Like genuinely. That was just truly insane.