Mrs. Jones, I think what your husbands father did, by coming to his son's brothers funeral, was very mature, and caring. HIs son was hurting, and he wanted to be there for him. It had nothing to do with your husbands mother. She is out of his picture, his son, and his feelings, and the death of a young man, his life cut short, are all he cared about.
My husband's ex-wife is weird, an alcoholic, and kinda nuts. 17 years ago, she sent my newborn a mobile for over her crib. My heart kinda melted, that she felt like she could send a gift. We now have a grandson, my step-grandson, in common. We are not best friends, but we can talk and laugh at holidays and birthdays.