This is just a theory from that panel of their feet as Ruri is saying they look nothing alike as siblings, but Ruri may not be pigshit's biological daughter.
Sometimes genetics comes into play on having "Morton's Foot," when your 2nd toe is longer than your big toe. Ruri's pretty much same length, while nameless son clearly has an emphasized longer 2nd toe. The kid who looks more like pigshit is probably his actual son. Ofc this is just a theory as that foot structure isnt always congenital.