a birth mark is different from a genetic disorder. its not coded into your genes, you cant pass it on.

as far as line/in breeding. i think youre alright for a few generations, but id try to mix it up when at all possible

and granted it would be possible for the wobble trait to become unlinked from the spider gene, as far as i know this hasnt happened yet and would probably be less likely than breeding 2 normals and getting and albino lol. so id say that the odds of getting a normal spider sib that wobbles are slim to none.