Having a disabled sibling can significantly impact the perspective the other siblings have, especially when it comes to social coping and education.  I am not certain as to what you refer to in terms of "methods of mixed design," but if you are speaking of research methodology design, it would indeed be very interesting to study children with disabled siblings - against a control group of non-disabled siblings.  Understanding their social interaction, maturity levels, communication abilities, and emotional development would be incredibly interesting.  From a personal perspective, I believe that having a disabled sibling is indeed a blessing, although it may be a blessing in disguise for some people.  Understanding how to communicate and understand others – even if cognitive or physical functionality may not be optimal – creates significant EQ skills that are applicable throughout all walks of life.  I think that the a counselor, especially for younger children, along with a support group, can help the entire family cope and overcome any obstacles that present themselves.