As a parent of a special needs child (my stepson has cerebral palsy thought to be a result of premature birth and complications at birth), you also should have your sister look into social security for her son. Depending on the level of disability, the income level of the family, and the age of the child, he may be eligible for benefits from the state (ranging from healthcare to income supplements).
For example, our son received full health coverage AND income when he lived with his mother, but when he moved in with us (in another state, and with a higher family income) he was initially receiving JUST healthcare coverage, then that was pulled based on our income level and cutbacks in the state (although he was still covered by our healthcare plan). Now, he's just turned 18, and as an "adult" will once again receive healthcare.. and is expected to receive income (although he had to turn 18 before that form was sent to us... and since he just turned 18 this past week, we sent it promptly back in, but that form hasn't been processed.) We're hoping the income is enough to have him BUY his own wheelchair equipped full-size van (or at least make a dent in the payments). At 18, he's grown, he needs a new and larger chair, and I'm not sure our minivan (which is getting old) will accommodate it (along with the rest of our family of five.. and especially in long trips).