Under their Children, press What are you trying to fix? and choose the option saying a child is shown with the wrong other parent. Easily sorted! Then choose the right one. For example, if your brother's son shows as his first wife's son but his mum is really his second wife, change it, and the boy stays joined to your brother throughout.
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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