I’m sorry about your sister and her things and Frankie. That must be incredibly painful.

This might be an unpopular opinion, but I think YTA, for essentially the reasons your husband is saying. Assuming Luke inherited your sister’s things when she passed (you called him your BIL, so I assume they were married), he was entitled to do with them what he wanted. Maybe he couldn’t cope with having them around, or allocating them, or seeing them in your house. It was a rash and maybe selfish decision, but he’s mourning the loss of his partner, and it was his prerogative. I understand she wouldn’t have wanted Frankie put up for adoption, but would she have wanted you to cut out her family either (including her kids, until they can drive or they find someone else to drive them)?