Its all very well talking about what the 19 year old should do; ie behave rationally - but people with depression are not entirely rational, because if they were, they would not have depression! Its like expecting the child with ASD to behave as if he were NT!
Look at the list of symptoms of depression on here: www.rcpsych.ac.uk/healthadvice/problemsdisorders/depressionmen.aspx
"Is depression really different for men?
Different symptoms
There doesn't seem to be a completely separate type of ‘male depression’. However, some symptoms are more common in men than in women. These include:
irritability
sudden anger
increased loss of control
greater risk-taking
aggression.
Men are also more likely to commit suicide."
NB - irritability, sudden anger, increased loss of control and aggression - could this be what is happening to this 19 year old? This is not a case of depressed parents cope, etc - as parents got into this situation, by the choices they made. We all know if we have sex, we could get pregnant; and we all know at the back of our minds that some children are disabled. Siblings most likely did not have any say in the parents' decision to have another baby; and even if their parents did discuss it with them, could they have made informed consent? I don't think so!
Its also being accepted that adolescence extends into the mid twenties, so while a 19 year old may legally an adult, it does not mean that socially they are, given the reorganisation of the brain is still going on; and people mature at different rates.
Having observed it myself, I'd say that a sibling can be at the end of their tether; and they simply don't have any more empathy, compassion, whatever to give! They are in a situation they can't cope with. There can be survivor guilt. While intellectually, they may know on one level all about the nature of the special needs; it can be really painful for them to hear on another level. I also think it is tempting as a parent, to want the other siblings to be "normal", because you think you have enough to cope with, and when problems arise in the home, to want them to leave because it makes life easier for the disabled child and yourself - when actually a sibling, with developing mental health problems needs more support; they are most likely to be suffering low self esteem, feeling unfairly treated, angry...Telling them to leave home is potentially another rejection.
Taking this to its logical conclusion, if parents say they won't tolerate bullying, etc in their house, what do you do - tell the sibling to leave? They then commit suicide? It may have taught them a lesson, but at what cost?