Can I buy my sister coffee drinks without our parents knowing?

My sister (14f) used up her weekly allowance and asked if I (17m) would buy her Starbucks and not tell our parents. I agreed to it and she got away with a free drink. So today her best friend also 14f joined us for the car ride and my sister asked again but this time her best friend wants in on the same deal. Am I wrong doing this without parents knowing?