If you have linked two people together by mistake — either as spouses or as parent and child — you can unlink them without deleting anyone.
Make the person you want to unlink the active person in the People panel, then use the Edit menu:
You will be asked to confirm before the unlink is applied. Use Edit > Undo immediately if you make a mistake.
Warning — Unlinking from a spouse also unlinks the person from any children they share with that spouse. If you need to keep the children with one parent, unlink from the other parent’s perspective instead.