I can use child to replace son or daughter, sibling for sister or brother, and parent for mother or father.
What is the unisex replacement for fiancée and fiancé?
I don’t need it for speech, but for formal or semi-formal writing.
For formal writing, betrothed would be my number one choice. But, as Lunatik already gave that answer, I'll provide some more: spouse-to-be (instead of the gender-specific wife-to-be and husband-to-be), future spouse, prospective spouse, intended (date and informal).
Of course, it's much easier to refer to use adjectives and refer to the couple as a whole, if you can rewrite it in this way. This opens the door to: engaged, affianced, pledged in marriage, etc.
My suggestion would be to use fiance (no accent). In my experience most people aren't aware of the distinction between fiancée and fiancé, since it isn't reflected in speech, and simply reverting to the shorter form and leaving off the accent (to confound francophones) may be sufficient.