little sister

My oldest has never been much of a social butterfly. In fact, since her little sister was born almost 5 years ago, she has often preferred to spend time with her rather than with friends when they ask her to play. The two of them are usually joined at the hip and do everything together. This week however a new family moved in around the corner, and Nia has left little Annie in the dust. I'm thrilled that Nia has a new friend, they have spent every moment possible together the last few days. However, I don't like that it is at the expense of Ann's feelings. What is a mom to do?