It appears that the closest relationship the tree will allow is between cousins. You should repost this with a '#feature-request' tag to bring it to the attention of Family Echo, as it would be a useful feature for fictional (hopefully!) or historical trees.
May I also suggest incorporating as many families as possible into this tree? For example, in 'The World of Ice and Fire', there are complete trees for the Starks and Lannisters as well, and the books give you information for the more recent generations of many other Houses.
Good luck, and Grow Strong!