Molly (1999)

Molly McKay is a profoundly autistic twenty-something woman who has lived in an institution from a young age following her parents' death in a car accident. When the institution must close due budget cuts, Molly is left in the care of her neurotypical, older brother, Buck McKay, an advertising executive and perennial bachelor. Buck makes her undergo an experimental medical treatment, with unexpectedly drastic results.