Whose fault is it?

My sister and I got into a fight, she gave me two dollars to go to Trader Joe's and buy her a bag of chips. I had $10 to buy my stuff, her $2, and $10 from my mom to buy ice cream. I got my food and the cost of that was about $5.50 and with the ice cream and my sister's chips the total cost was about $13.50. I gave the cashier the two tens and kept the two dollars with me. Even though some of the dollars were mine I just have them to my mom and kept the change and two dollars. When we were driving my home my brother got hungry so I told him to have my sister's chips and she told him to have mine. I explicitly said "Have *****'s chips" the 's signifying that they were her chips. But he wanted some of mine and I let him. Before when my brother was sad about not getting anything I told him to ASK my sister to share with him. Two days later my sister opened her chips and shared with him, over the course of time that she ate her chips she called them 'My chips' not 'Our chips' or 'The chips' meaning she knew they were her's. But now about a week later she tells me that I owe her her two dollars back. I told her I used it to buy her chips for her and then she claims that she didn't know they were her's even though she called them her's numerous time and I explicitly said they were. It's not my fault she thought wrong. She said that she thought I used our mom's money but that's not possible if the cost is over $10. She expects me to give/owe her money since she shared with our brother (I did too), so who is that fair? Whose fault is it? Mine or her's?
Asked Sep 07, 2017

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