it almost sounds like shes jelious or envious or intimated or threatned or resentful or some other sort of "grudge" that she sees herself "less" than you in some sort of way.
having you around she thinks only mannages to draw attention to how inferior she icompares to other ppl.
and i mean no offence by this but it is probably even a worse deal when the person she is inferior to is mentally challenged in some way when she is the one who mind is normal .
maybe try not to let it get to you so much and just shrug it off as its her problem and oh well whatever .. blah blah blah .. talk to the hand cuz you aint gonna bum me out listening to you play pity party with yourself.
maybe if she sees not upseting you by doing it she will give it up and quit doing it.