Originally Posted by Anark
That's actually one of the things that gets dealt with in the series 3 opening episode with the Star Wars-style titles.
Apparently, he gave birth to two boys, Jim and Bexley, but because they were from a different dimension they grew really fast. They were the size of 18 year olds three days after birth. So Lister sent them back to their dad (the female Lister) in the other dimension so they could have a normal life, sort of.
I forgot all about that.
I never minded the lack of continuity. If anything, it allowed them to not write themselves into a corner.