Your relative may know more about deceased family members than they are willing to tell you. And they may never tell you everything you know, no matter how much you wish they would or how many times you ask. For reasons that are entirely too long for a "short tip," I know my own grandmother knew more about her grandfather than she ever told me, including the fact that he had a second wife. Yet my queries about him always received a "don't know anything response."
Sometimes that is all you are going to get and sometimes you have to let it go to preserve relationships with your living relatives.