You can edit (create) family ties in SimPE. Just remember to use the backup facility in case you cause a spontaneous family combustion
This is the Larson family that I'm using to give you an example to work from, I've first selected jason in the left-hand panel. Kate Larson isn't actually related to Jason, so I've right-clicked on her in this panel and added her to the family.
She defaults to being Jason's new sister, but I could also make her a child, parent, or spouse.
The 'Keep Consistent' checkbox on the bottom-right is to make sure that when you commit changes, the relationships work in both directions.