In a mathematical sense, the opposite of "X" is "not X" and this works in all cases.
But in language text books, or in common usage, there is a lot of ambiguity in "opposite".
Father ~ Mother
Son ~ Daughter
Here, both are with reference to the speaker, meaning "my father" ~ "my mother".
Husband ~ Wife
Here , the reference point itself changes, meaning I can not have both husband and wife.
More-over, why should only the sex change for opposite ? Can Son be the opposite of Father ? Considering reversal of both characteristics, we should have Father as the opposite of Daughter, in line with the Dictionary meaning of opposite "Being directly across from each other; facing" or "Characterized by opposite extremes; completely opposed" [from wordweb].
Friend ~ Enemy, Acquaintance, Stranger ?
Uncle ~ Aunt, Nephew, Niece ?
It seems more logical (unambiguous) to use the mathematical sense : opposite of "Father" is "not Father".
In language learning books, what does "opposite" really mean ? Is it some arbitrary colloquialisms ? Is there some unambiguous meaning ?