Well it certainly can't be a primary source unless it's material the author had witnessed him or her self. So if you see, "I remember how happy they were were when they married, just a week before my tenth birthday...", then that would be a primary source. Otherwise you're talking secondary sources. But it's still a source, if not an ideal one. I have a very large book which is essentially entirely concerning the descendents of a daughter of my 5ggrandfather David Evans, Sr. The author says in the introduction that he has at least two references for every fact in it, but he didn't have them arranged to include them. So I'll cite the book as a source, but will try finding other sources to verify his information. IOW, a source isn't the same as a proof. Just try to make sure you have enough information for the person who want's to verify the information to get to it. But getting back to your original question, no, I wouldn't use it by itself to make you confident about a connection or fact.
