Some of what you said wasn't entirely clear (re who gets slapped for what) but ...
Why wasn't she disciplined when she did something wrong? Just because you don't mean to do something doesen't mean there arent' concequences. A child running across the street after her ball dosen't mean to get hit by a car but her parents will certianly yell at her (or other discipline measures) to help teach her not to do that. If she's never taught or allowed to experience the concequences of her actions (with her older brothe protecting her), she's going to have trouble as she gets older.
And, in the culture she lives in, how are the mentally disabled treated? Is she always going to be a child who is taken care of by others and never responsible for her actions? Which would make what her brother is doing a normal part of the culture - not being condencending. If, OTOH, the culture is more severe than that, her mental disability - whatever it is - would not excuse her actions and she might be treated like an adult, whatever her mental abilities to behave like an adult actually were.
Basically, IMO, to sort out the character you need to determine if she has a disability, if it makes it difficult or impossible for her to behave 'normally' and if her family might be sheltering her because of her ability/disability. Her parents might have been lenient, her brother overprotective, because they imagine she cannot take care of herself.