If my granddaughter has an eating disorder which is fueled by her step fathers verbal abuse and silly demands, do I have any rights to step in?

Surely, there is much more to this than one sentence. You need to have a full discussion with a local family attorney. If the child is in danger, you do have grounds to step in, not necessarily rights, but reasons to file perhaps a temporary custody case, and if the parents will not consent, then you would have to prove by clear and convincing evidence that the parents are unfit.