If you edit your tree online, please read the following article: How do I add adopted or step children to my online family tree?
In Family Tree Builder children are connected to a husband/wife family group. That means that when a new spouse is entered, the children of the former marriage stay with the former family group and are not shown as belonging to the family group which is made up of the new husband/wife.
In order to show all the children under one family group, use the “Attach Child” feature. This feature designates to which family a child is connected (not necessarily related) and therefore where she/he is displayed in the tree.
1. Right click on the parent you want to attach the child to:
2. Select the Children > Attach Existing Child menu:
3. Select the child from the list of people in your tree:
4. Select the type of relationship this child should have to the new parents: