Good question that I've often thought about. I think if you reunited a mother and pup shortly after the pup went to a new home, they would remember, but I think as time goes by, not so much.
Now litter mates I think are a different story for whatever reason. My aunt has Alice's litter mate brother and they've been seeing each other pretty much weekly since we had them. They both like all other dogs and play with them, but when the two of them are together it's CRAZY lol. They roll around on the floor and play bite and growl with each other, just like puppies do when they're still part of the litter and with mom.
I've seen it other times too - my cousin's beagle mix has a close friend who has one from the same litter. They don't see each other often (maybe about 10 times in their 2 years. of age) and they FREAK (in a good way
) when they see each other and play like puppies. All my cousin has to say to her dog is "where's sister?" and her dog goes nuts barking at the door lol.
I tend to think they're more attached to their peers by the time they're ready to go to a new home than they are to mom at that point (she's "cut them off" by this point with weening, so I think they move on lol).