I had a single chick born to one of my broody hens, then three weeks later, another hen hatched three babies. I was worried about having to raise a single chick once her broody mom decided to go back to the adult flock, so the day after the three babies were hatched, I slipped the single three week old chick in with mom and her three one day old babies after dark. I carefully placed the three week old baby under Mom's wing so that she would be warm through the night.
The next day, I watched closely and Mom accepted the three week old chick along with her two day old babies. No problems at all -- everybody blended together into one big step family.
Mom has since gone back to the adult flock (and after a month of being a normal adult, just today, she announced that she's wanting to brood AGAIN!).
Meantime, the four babies are growing up together with no problems whatsoever. The little three are almost two months old, the older girl is well over two months old.
I don't think they realize that they are not of the same hatch.