What are some documents I can quote and cite in order to convince my relatives that refusing to attend mass is a mortal sin and that confession is obligatory in order to receive God’s forgiveness?
They don’t seem to want to listen to people that would say otherwise perhaps I can find a way to explain these things to them.
As a correction the Sacrament of Confession isn’t the only way to receive forgiveness–it’s the only ordinary, guaranteed way. You’ve several ways other ways: private remorse and repentance, confession to another person, charitable acts, and even taking the Eucharist, I’ve heard.
How did this conversation come about with your relatives (who I assume are Catholic)? Did they ask for the Church’s position, Biblical verses?
If they’ve been dismissing you repeatedly, I’m afraid you may be talking to the wall.
The catechism, Bible, Church authority, etc., mean nothing to those who don’t accept it; whether they’re in the right or in the wrong is besides the point.
Pointing out the wages of sin is, IMO, the wrong way to go about it, especially because you suggest they’re not educated in Catholicism.
Rather, you should convince them there’s a positive motivation to attend, e.g., the Mass and Sunday are a way for Christians to relax and focus on God.