Explained in a Reddit thread:
That is a common scam. I used to be a bartender, and every once in a while that would happen. The person just has to put something "impossible" on the bill, and Visa will not process it. Since the amount and the tip are hand-written, it means you had to enter it into the till with the tip amount AFTER the sisters had left. Visa would outright reject it then as almost no one has that much credit. Even if Visa accepted it, the customer would just call later and claim she had been scammed. Visa would annul the entire bill. The only thing you could have done is pointed out her error, which is why she "secretly" and repeatedly kept asking you not to talk to her sister about the tip. That and the fact that she added 200K to 111 dollars and came up with 211K.More at the link.