you might find that the younger sow gets a lower place in the pecking order, but that shouldn't be a major problem. try the usual methods of bonding- get them all out for playtime and distract them with food, maybe disinfect their living areas if you think she may feel intimidated. if it's a large group you could try getting her out with one or two of the less dominant sows first, and build it up until she's used to all of them.