There is currently no way to directly show an adoptive relationship, although this is a very common issue so Family Echo may change this in the next update.
I've found that a good way to display the information is to add a partner box to the adopted child, and give it a name along the lines of 'X's biological family from here', then to 'grow' the tree from that box. As the adopter is the uncle this shouldn't result in too many duplicate boxes.
Hope this helps!
