I saw an amazing pair of expensive big sunglasses that I will never wear. I'm thinking of buying them anyway.
Really? And how are you going to afford them?
My sister gave me her card for groceries, but this is a better purchase.
If she gave you the card for groceries, then use it for that purpose. Buying sunglass with her card is wrong.
It's my money, she gave it to me for the groceries
She is expecting you to buy food, which is a necessity, with that money. She'll be upset to find out you have sunglasses and no food. Just use the money for it's intended purpose, before you start an argument.