I believe that the younger the original pup is the easier it is to introduce a new brother or sister.
There are several members that have introduced a new pup when their original pup was several years old, so it is certainly possible to do so, just might take a little longer to get them comfortable with each other.
when i knew jumba was going to be mine, i started bringing chiwi's snuggle bag with me every day to work (i work at the breeders so it was easy) and every day i'd have cuddle time with jumba, letting him sleep in her bag while i held him. then when i would go home i'd give her bag to her and she would just smell and smell then fall asleep. when i brought him home i just popped him right on the floor with chiwi and that was that, she loved him and they barely even sniffed each other. it was like they already knew each other.