What should we do if our daughter started misbehaving after being teased?

my daughter, gemma, has a developmental disability and is mildly mentally retarded. she is fourteen but has the mild of a little girl. she made friends with a five-year-old girl who lives a few houses away, the best friend she has ever had. her name is josie. they played together in the park everyday. gemma was beside herself with joy. i was pleased that she had company, but i was also worried. josie will grow older and she will look at gemma through different eyes. i braced myself for that moment. oneday i was cooking in the kitchen, when i heard screaming from outside. i peeked out the window. josie`s older brother, who is almost nine, was circling gemma on his bike as he shrieked at her, calling her stupid, a baby, a freak and a bunch of other nasty names. josie was skipping along at his side, a savage smile on her face. gemma stood horrified, her face burning red. josie danced about, hurling contempt. then they peeled off, howling. gemma came inside and sat in silence all day. it was not fair and gemma was really angry. she started misbehaving. she threw things aside, ripped many books from the shelf, and would tease her little brother, charlie, until he cried. whenever we dropped things or tripped, she would laugh out loud. gemma would binge on chocolate milk, cookies, ice-cream and lollies all the time and refuse to touch the fruits and vegetables i beg her to eat. things started going from bad to worse. bedtimes became the most difficult of all. she would be so loud that neighbours came to complain. gemma`s behaviour was driving my family crazy. she even acted up in school, picking fights and throwing huge tantrums whenever she could not have everything her own way. does anyone have advice on what we should do?