Yes you can usually keep female littermates together, even once adults. Many girls even those not related like to be with a friend.
Males are a whole difference storey. Male baby litter mates usually do fine together while waiting for a new home, but problems with boys usually start when puberty hits.
I always kept each baby in a separate cage before going to their new home because it lessened the stress on them and allowed me to watch each baby for a few days to ensure he/she was eating well, gaining and not too stressed from being by themselves without mom or sibling.