HELP PLEASE NEED ADVICE?

Ok it sounds bad but you have to understand the back story. I work for a very small business in a tourist town only 3 employees and a boss. So for the last like 2 years one employee and the boss who also happens to be my uncle have been taking advantage of how nice i am and always ask me to work for them so they can go hang out with friends or go shopping. You need to understand that this happens ALL the time. In the last two weeks this one employee has asked me to cover his shift 5 times out of 6. So today I ended up hanging out with some friends which I NEVER get to do since i'm working for everyone else and I realized that I wanted to hang out for the rest of the day. Well the employee says he cant work. And my boss/uncle already asked me to come in way early yesterday again so he could hang out with his friends today....so what do I do? I know it would be responsible of me to just work but i'm responsible every other damn day of the year...I cant have just one day?