I'm going to assume that hopefully this isn't actually YOUR family tree but some kind of fictional or other one, as incest can have lots of health defects on the results of it.
Let's say you want to make siblings have an incestuous relationship. You would create them, then go to the toolbar to access the options and details of one of them. Then click on the bottom option, "Partner with person already on tree". It should show a scrollbar where you can pick the other sibling from a list of people already on your tree.
However, it should be noted that you can't make such a relationship with direct descendants or ascendants. No parent-child relationship, no grandparent-grandchild relationship, no greatgrandparent-greatgrandchild relationships. But feel free to let siblings, aunts and uncles, cousins and the likes to sin as much as they want!
If you want to create some good old incest with a parent and child, create a partner for the child but fill it with the parent's details. It's not ideal, but close enough. Hope this helped!
