Mom and Dad are not really able to grasp their financial situation fully and rely on me, as the general power of attorney, to handle all their financial details. My middle sister and I may speak with a financial advisor but its pretty clear cut that there is no other way to increase my parents available cash other than to reduce this stipend. I don't believe there can be any legal pressure brought to bear on this. Its a very painful family matter.
It is painful, and you're in a tough spot by being the power of attorney. Your parents obviously trust you, but at the same time, they're enabling your sister and counting on you to continue to support her.
When you say that your parents can't grasp their financial situation fully, what do you think makes them unable to understand? Would it help to have your financial advisor speak with them? It sounds like they cannot continue to support your sister, as much as they want to.