I had two accidental breedings of Shep/Labs mating with Shep/Labs. I am not a breeder and both were heat cycle/spay timing related and occurred when I was not there to protect 'things' like that from happening.
After the 7 pups from the first breeding were no longer with us, the mom 'adopted' my older son as her 'new baby' and after the 3 pups from the second breeding were no longer with us, the mom 'adopted' my younger son in exactly the same ways.
"Once a mother, always a mother" is completely true...at least it has been in my experience. lol
However, I have brought in older and younger babies to both cats and dogs, and I find that there is an 'age break' in that the puppies (or kittens) that females will 'adopt' are very young, whereas over about 3 months it was always my males (felines and canines) that took the older babies under their wing and taught them everything. I have seen this behavior in the animals since I was younger than my teens.
I also think it is a great idea, if you have a lactating mom to allow her to help out with motherless fosters. I don't know if I could do it, though...I'd have a mess of babies growing up adopted at my house if I tried 'cuz I'm great at fostering and suck like rocks through a straw at giving any of my fosters up.
