The birth of a new baby is only one example of a new family member. When older brothers and sisters marry, they may bring their wives and husbands to live with the family. Relatives, a cousin, or a grandparent, may come to live with the family.
This will require the old and new members to make some adjustments. New division of household work, new routines, new schedule. These adjustments are necessary to make things for both parties a lot easier.
Adjusting to a new member is no easy job, but once you get to being with the new member. you will realize that having a new members is fun and interesting.