Make a story that you Sponsor's financial situation has changed suddenly because of sudden unexpected events like natural disaster or diabality or disease that was not expected at the time your Sponsor wrote you a letter. The circumstances cannot be a general or predictable in nature such as your Sponsor sending your brother to a medical school. In my friend's case, he made a statement that a river near his house got flooded and became a threat to his house which was not predicted before. Now they needed to build a big construction to protect their house, so his father was no more able to sponsor him. He got some papers from a Civil Engineer from Nepal to support his claim. He had his EAD approved. As suggested by other, you will need to go see your advisor before you apply. Make a valid reason and try to back up with document, you will get it for sure. Good luck dude. Puruz