If you really don't want to miss your sister's wedding, i think you can get it stampped in Mexico. They do have visa dates available at a sooner date than Canada. But when you go to mexico, make sure you take all your documents. A couple of my desi friends were denied with their H-1 visa as they forgot to take one letter from the employer. Hence they had to go back to India. It is a lot stricter in Mexico. But if you don't want to take any risks, then just stay in the US and send your sister a big gift with all your love. She will understand your situation