Just remember a secondary source isn't necessarily wrong. In 1907 a widow testified as to who the siblings of her husband were. Did she know they were her husband's siblings because she had first hand knowledge of their parentage? No. She had been told who her husband's siblings were. Did she have reason to doubt it? Probably not. Was she wrong. In this case that's not likely. She was suing her husband's family over her inheritance and the chance that one of her husband's siblings was left out is fairly slim.
It's not 100% proof she was right, but any source needs to be kept in context. She's a secondary source of the relationship because she was not present at the births of her husband's siblings. That doesn't mean she's incorrect.