‘A # B’ means ‘A is the brother of B’.
‘A @ B’ means ‘A is the daughter of B’.
‘A & B’ means ‘A is the husband of B’.
‘A % B’ means ‘A is the wife of B’.

If S % D # F @ G & H @ J, then how is F related to H?

Let's decode the relevant parts of the expression to establish the relationship between F and H:
G & H: G is the husband of H. (⟹G is male, H is female).

F @ G: F is the daughter of G.

Since G is married to H, any daughter of G is also the daughter of H. Thus, F is the Daughter of H.

(Note: We do not even need to decode S% D#F or H@J as the relationship between F and H is fully defined by F@G&H.)