Half siblings
Homer Simpson has a half sister in England named Abbey, born from a relationship between his father Grampa Simpson and a certain Edwina.
To represent this situation in Family Gem you have to end with two families, both with Grampa Simpson as father.
One method to add a half sibling is to activate the advanced tools.
Then Diagram → long press on a person → Link new person / Link existing person, paying attention to the Inside option that let you choose in which new or existing family add the relative.
Another method to obtain the same result is to create the two families with ☰ → Families → 
Then inside each family add the members with
Maybe we only want to add Homer's half siter Abbey in the tree, but not her mother Edwina.
Well, it's simple: leave the first family only with Grampa Simpson as parent and Abbey as child, without Edwina.