Well, first of all, I think you should refer your question to Ask an Apologist. But I would say that it would only be sinful if there was intent to shop on Sunday. If there was no other way the shopping could have been done on another day, I don’t think it was sinful either. You should not shop on Sunday when you don’t have to because it violates the Third Commandment: “Remember the sabbath day, to keep it holy. Six days you shall labor, and do all your work; but the seventh day is a sabbath to the Lord your God; in it you shall not do any work”
The Catechism explains:
“2187 Sanctifying Sundays and holy days requires a common effort. Every Christian should avoid making unnecessary demands on others that would hinder them from observing the Lord’s Day. Traditional activities (sport, restaurants, etc.), and social necessities (public services, etc.), require some people to work on Sundays, but everyone should still take care to set aside sufficient time for leisure. With temperance and charity the faithful will see to it that they avoid the excesses and violence sometimes associated with popular leisure activities. In spite of economic constraints, public authorities should ensure citizens a time intended for rest and divine worship. Employers have a similar obligation toward their employees.”
Whoever committed the sin, it’s not a grave sin, probably a venial sin. But we are called to live by example. A Christian who buys something he or she doesn’t need to buy on a Sunday is setting an example. It’s not worth getting in a fight with your parents over this, but mentioning it and explaining this in a kind and respectful manner would be good for everyone, I think. I’m 25. That’s what I would do.
But again, I’d encourage you to ask an apologist.