Almost 15 years younger...and that was considered old for bearing children and it was after a lifetime of having children every few years. For someone to become a mother (for the first time) at age 50, it would be very dangerous. Also remember that most people lived only to about 65-70ish naturally, too.
I realize this is a role-playing game in a fantasy setting, and I sound ridiculously nitpicky here, but I guess I like my fantasy bound by some realism...
So you can either have an NPC parent (which is what you were going to have anyway -- just a different name) or, if you absolutely have to have a 2nd gen kid, you can change their last name to Delancy and make them the kids of one of the brothers. Or you can add 10 to their ages.