How to find Brothers and Sisters?

Suppose Adam has children named John(male), Jack(male) and Mary(female). What schema/query finds John's brothers and sisters based only on direct relationships to parent and gender. A direct relationship would be parent/child. An indirect/derived relationship would be brother/sister. The query is not to use Adam directly. Below is a solution using dbd.

(new 'male 'gender)
(new 'female 'gender)

(new 'adam 'person)

(new 'john 'person)
(set john gender male)

(new 'jack 'person)
(set jack gender male)

(new 'mary 'person)
(set mary gender female)

(new 'child 'verb)
(set adam child john)
(set adam child jack)
(set adam child mary)

(; Get john's brothers

    by getting persons
    whose gender is male
    and is child of john's father
    and that person is not himself)
(; Gets jack)
(!= (and (get person instance *)

            (get * gender male)
            (get (get * child john) child *))
     john)

(; Get john's sisters

    by getting persons
    whose gender is female
    and is child of john's father
    and that person is not himself)
(; Gets mary)
(!= (and (get person instance *)

            (get * gender female)
            (get (get * child john) child *))
     john)