Siblings always are a nice addition to a maternity photography session. They show how the new child is becoming part of the family. As a photographer I will discuss the shoot with the future mother before her appointment. I suggest that the siblings be brought to the studio for the last part of the session. This allows the creation of the rest of the photos without having young children getting bored waiting. Which in turn will result in better photos with them.
I often will catch a moment when everyone is just being themselves. These photographs tend to make very special maternity memories. I call this shot got your nose.
This type of playfulness is just the kind of memory one wants to remember.
My job as the photographer is to always be aware and to bring a comfort level to the maternity session that allows for true family interaction to be captured. You can easily see how much fun is going to be had with this new baby.