Right now there is no way to do that. You would just have to note in the biography section that his adoptiove Mother is ______ and she is is biologial paternal Aunt.
This is a question/topic that has come up a lot and is one of the more requsted ones. FamilyEcho said they might someday make it it possible that one could show a persons Adoptive/Biologoical parents at the same time. However, ight now its not possible, a person can either make a note in the persons bio secetion or make two seperate trest one for their gentic family/adoptive family. In your case your father being adopted by a gentic family memeber then noting in the bio secetion right now would be the easiest way.