Is It is SIN to Work on Sundays?

No, because Christ is our rest according to the Gospel of St. Matthew 11:28-29. And on Sunday, we attend mass as to receive our rest (i.e., Christ) in the holy Eucharist. This does not mean, however, one may avoid his Sunday obligation for mass, according to CIC 1247: "On Sundays and other holy days of obligation, the faithful are obliged to participate in the Mass. Moreover, they are to abstain from those works and affairs which hinder the worship to be rendered to God, the joy proper to the Lord’s day, or the suitable relaxation of mind and body. " Missing mass on both Sunday or Saturday evening is violation of the third commandment (i.e., keeping the Sabbath).