It's not only a distraction from his own thoughts, his own grief. It's honestly been about as hard for him to dredge up something about Mary as it is to get Riku to divulge much about himself to those who don't already know something about him already.

Which...

It's why he doesn't fault her that.

"Those sound more like titles than names," he observes, thoughtfully, "...I never had siblings. Hey, would you tell me about them?"

He never had a sister or a brother, not until Dawn became Dawn, and he struggled to find a way to define what they were to each other. Something that wasn't just a copy and a "real thing".