"but is it reasonable for her to deprive her child of the support and help of someone he loved just because that person said something stupid?"
Yes, you see, here is the crux of the problem. What is said is hurtful, tasteless and beyond the privilege afforded to your sister in this case. The child might have loved the person your sister portrayed and your sister might have even been the best intellectually disabled volunteer walking planet Earth. She may not have even meant it in that light. However she disowned all of that by making hurtful comments about her own work, that she could even mouth the words to see them in that light is the problem. So, the child may have been deprived of the most awesome disabled volunteer to walk this planet, but it was your sister whose actions caused that behavior. We all make mistakes, and mistakes have consequences. Some more grave than others. It's a lesson most children learn early, some a little later than others and in this case a lot later.
Is the mob always right? Of course not, but in this case and your sister's.. spot on.