I like learning about people, but I would only ask “how’s your relationship with your mother/father?” if I already think the person’s unusually open. It’s a pretty direct “therapy”-style question, and some people don’t like feeling analyzed that way.

But it’s not that hard to get there a little less directly. If you just ask about people’s days and weekends, their parents will come up at some point. Then you can ask more gradually and organically.