Are you trying to represent all of the relationships in a family tree – that is, a pedigree? If so, there is not a way to do so in the OMOP common data model. There have been discussions of a genetics specification in the OHDSI forums, for instance here and here, but they seem to have left out representation of pedigrees in the draft specifications I have seen.
The standard way this is done in the field of genetics is to have a pedigree file where each row has a pedigree id (to represent membership in an extended family), a person_id, father_person_id, mother_person_id, and sex.
One could augment the OMOP person table to add father_person_id, mother_person_id fields, and one could represent the whole family tree (I would leave out the pedigree id). One would have to add rows to the person table for relatives to the table, though they may not have clinical information, as well as set 0 to the father_person_id and/or mother_person_id, when you reach high enough in the tree that you don’t know the ancestry.