Have you asked your sister what she wants for her son? You feel strongly he should be in a residential placement, but she might want him to stay at home with a good support package. I know you are concerned, but spend some time listening to your sister to find out how she feels rather than keep pushing the residential care option.
The reason your sister is so emotional about this is because it is her child and you are talking about her very vulnerable, dependent child leaving home and being looked after by strangers for the rest of his life. As I am discovering, it is a head fuck. However, I do feel it is her duty to her son to get a grip and start finding out what is out there for him, not yours.
If he has severe autism, he will probably have a social worker and a Transition worker. Both workers are meant to help parents and child through the process of moving into adult care/education/health, i.e., 'Transition'. Transition is meant to start in year 9, that is, when the child is about 13 or 14 years old, with a review, resulting in a plan that is meant to look at the child's and the parents aspirations for the future.
What often seems to happen is that parents are not really guided much at all and then panic decisions are made just before they turn 18.
Go to www.learningdisabilities.org.uk/publications and download the leaflet 'Prepared for the Future'. It is a general introduction to transition into adult life. Be warned though, the government is making sweeping changes to all sorts of things, so some information might be out-of-date. He will need to fit the local authorities criteria for help too, which is now increasingly set at 'critical' or severe. The Challenging Behaviour Foundation also has good publications and advice for parents of children with severe learning disabilities and challenging behaviour