She is probably still on the streets because she is intellectually handicapped.
1. As long as she can take care of her physical needs she won't be institutionalized.
2. Her handicap may make her be concidered incompetent to stand trial for any offenses.
So unless she is concidered a physical threat to herself or others she will remain "on the streets." (verbal abuse doesn't count.)

I know this doesn't help you out much, but the same laws keep many of us from being institutionalized. "Going off" on her is maybe the only way to reach her to say "what you are doing bothers me". Avoiding her is a solution, but don't change your live to do it.