Ok. Let's assume the brothers are already part of the tree and you are adding the sisters as their wives.
Select brother #1 and then click on the [Add partner/ex] button to add the info for sister #1.
Select sister #1 and click on the [Add parents] button and add the info her parents.
Select sister #1 again and click on the [Add brother/sister] button and ad the info for sister #2
Finally select brother #2 and click on the [Add partner/ex] button then click on the [Partner with person already on tree] button. Find and select sister #2's name in the drop down menu then click on the [Add as ...] as appropriate.
