Post Doc referees related to one another
I don't see any problem with listing them separately, just as if they were any two other people.  Of course it is good to have an additional reference who is not related (and from a different institution).

I think it would be inappropriate to omit one of them, or lump them together with the other, solely because of their family relationship.  It would look like you are devaluing their professional opinion for reasons unrelated to their professional work.