Really? I think I've read all the C&H strips multiple times and never picked up on that. Notably, Calvin's parents are never given names, and the only relative ever featured as I recall was his uncle on his dad's side.
Do you have a link to a strip where this is hinted at?
IIRC Watterson said that he had been planning to introduce more adult relationships to the strip, but he realized this would be difficult when Uncle Max couldn't address his brother (Calvin's father) by name.
It's made clear both parents were naturally difficult too, which is part of the point. This panel probably had to be expressly stated since Mom doesn't encourage Calvin's (over) imagination like his father does.
