Upvote:-1
You can ask at the consulate, but apart from the timescale, you'll have to prove to them that you're not using this visa as a backdoor to enter the USA illegally. Going to "visit your sister" is something they've probably heard many times before.
Probably easier/quicker for your sister to visit you in Canada.
Upvote:7
You can apply for a US visa at a US consulate in Canada. You cannot get one at the border.
But processing times in Canada can be very long.
For example, the wait time for an appointment in Toronto is 70 days. Calgary and Quebec city are somewhat more reasonable at 14 days.