I suppose you are trying to build your tree on FTB.
To add your siblings just open you tree, and your card there is edit. Click Edit>Siblings> Choose add sister or add brother etc.
You can add children the same way.
As for grandparents you should see empty boxes above your parents that say add mother and add father you can add your grandparents from here or you can add parents to your parents again from Edit on the person card.