If I interpret your question well enough: you want to add children to your parents, right? So you are one child already present, and you want to add 'siblings'.
For that, there are two paths.
1. You go to your own profile, edit. Then you want to 'add a sibling'. That option is on the right hand side of a profile. That will assume the same parents as you have.
2. You go to the father, edit, and from there 'add child/children'. Fill in the information and make sure to choose the mother correctly. That is the option/question in the bottom of the screen. Without chosing a mother the child will only be attached to the father and be a 'half' brother/sister.
Hope this helps.