I just answered this same question someone else asked, hopefully this will help.
Say you've got John and James, brothers. And Anna and Sally, sisters.
If you start with (for example) John and add his brother James, then you add John's wife Anna, and then add her sister, Sally. Click on Sally and click 'add partner with someone already on the tree' and then find James' name. The system will probably automatically add some duplicates depending on what person you're clicked on, but it saves you a lot of work and stuff