Under their Children, press What are you trying to fix? and choose "A child is missing". A new leaf – how lovely! Add them as someone new or pick them from the tree, and choose the other parent. For example, add your sister's new baby Ava, with her date of birth if you have it. You can also use the golden + on a card to add a child straight away.
Where to find it
Person page (Safe edit on) → under Parents, Partners or Children → What are you trying to fix?
Only people with the "Add people" and "Remove" ticks, with Safe edit on.
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