Family latcher snatcher

An in-law who has few, no or strained relationships within their own family but gives you advice and direction then builds alliances and bonds with your kin where you feel like an outsider.

My dil used her family latcher snatcher skills to alienate me from my siblings.

Roll Hog

Someone who is wrong but will not admit it

The Roll Hog couldn't recite the alphabet backwards but insisted he did.