I would suggest to stop thinking of sin as a “not to do” list. Anything that prevents or hinders the opportunity to express or experience goodness and charity should be avoided. The severity of any fault depends on the severity of the matter, free will, and knowledge of the act.
In your case (well mine too) yes there is the forth commandment “honour your father and your mother” however as a Christian you also have Jesus’ teaching “if any one loves father, mother…more than me they are not worthy of me” (paraphrasing).
So I would obey them but also have the courage, calmly and with charity, to tell them that I believe that what they are doing is wrong and wish not to take part of it any longer.
I am guessing you are in your teens and wish to earn a bit of money on the side and weekends are the only times that this is possible. Honestly I do not see much wrong with it as long as you still attend Mass and make special effort to make up the time during the week i.e. prayer and reflection. Some people will disagree and say Sunday is to be work free but sometimes people’s circumstances prevents us from doing this. Ofcourse if you could do with working Saturdays instead of Sundays then that is preferable.
Hope this helps
Alex
