Add the first couple, say George and Doris, as normal if they are not already on the tree. Then add, Anna as George's sister and Karl as Doris' brother. On one of them, say Anna, click 'Add Partner', then 'Partner with someone already on tree'. Then you will need to find Karl on the drop-down menu. If George and Doris are already on the tree, just use the 'Partner with someone already on tree' button for them as well.
You will find that the tree will display 'Duplicate' boxes (you'll see what I mean when you've added them). This is normal and only one copy of the information is held still. If you want to know more then look up any questions relating to incest (yes, I know), as this is a fairly common sort of issue, although it doesn't make the tree harder to use in any way.
Hope this helps!
