Demanding money from widow

My brother in law got some payment(1 lakh) from individual for some business. My BIL got this money transacted directly from individual to my sister's account on Jan,2014. My sister does not know about this as her my BIL was handling her accounts etc. A year later my BIL died in April 2015. Now the individual is demanding money from my sister. Individual is claiming that my BIL did not completed the business work. Is my sister liable to pay back the amount. 1. She is widowed & have no source of income. 2. She has two little girls & her inlaws not supporting her. 3. Though money transacted to her account but she does not know about it until it was demanded. Also she did not use it. It was handled by her husband & mostly account was Nil balance. 4. There is not reciept of money from my BIL given to that individual not as a payment of advance, 5. There was no written agreement about business transaction. 6. It may be possible that my BIL completed work & individual is claiming falsely. 7. Is my sister legally liable to pay back this un-investigated amount merely because it was sent to her account? Kindly help/advice.