I see no mention of family beyond siblings. Was he not married/in a relationship? I realize some people don't want that, but it's always made me sad even so.
He reportedly was never in a relationship. This comes from unidentified members of the Ritchie family who set up a web site focussed mostly on his unpublished PhD thesis. The site includes some personal information on his life. From https://dmrthesis.net/story-and-question/ (near the bottom) "In his adult years Dennis had a reputation for being famously private.... There were topics that were completely off limits wi…
More broadly:
"As a youth Dennis was social and outgoing, then in young adulthood he transformed quite rapidly to become more private and anti-social. Hard to pin down exactly when this would have been happening… except that clues from John and Lynn’s interviews suggest perhaps sometime around February 1968 might be central."