Assumptions (change names as desired):
Alice and Betty are already in the tree as sisters
Charles is already in the tree
Charles is partner with Alice
Charles & Alice already have a child together
What to do:
Select Betty; click Add partner/ex
Select Partner with person already on tree
Click drop-down and select Charles; click Add as ex-partner
There is now a line between Charles & Alice and between Charles & Betty
Select Charles; click Add child
Select Add child with Betty
Add child info as usual
If you need to start over, select Charles and click Change or delete, then click Change partners, then click Remove ex-partner Betty
