No matter how often you ask or how many ways you try to get an answer, some people will not tell certain details about themselves or their close family members.
There comes a time when you have to realize that a relative may know things that they simply are not going to talk about. Period.
My own grandma knew her brother was divorced for several years before he was killed in a car accident in the 1930s. She never told me. She also knew about her step-grandmother and where she was buried. She never told me that either. Other things she readily shared.
But there are times when people won't tell you certain things. If it's a close family member, consider whether or not it's worth straining your relationship to get that one detail.
Especially if you think they'll never tell you anyway.