Oh my, I posted this response before seeing Father Ruggero’s response. So only take my advice in light of what he has already posted. 
This is an excellent question, Upgrade.
I applaud your conscience and it’s sensitivity! You have a great desire to know and follow the truth!
I have found it helpful to find answers from the Catechism or a knowledgeable and trustworthy priest rather that seek opinions from fellow Catholics. However faithful they may be, their opinions can vary widely.
According to the Catechism:
2217 As long as a child lives at home with his parents, the child should obey his parents in all that they ask of him when it is for his good or that of the family. "Children, obey your parents in everything, for this pleases the Lord."22 Children should also obey the reasonable directions of their teachers and all to whom their parents have entrusted them. But if a child is convinced in conscience that it would be morally wrong to obey a particular order, he must not do so.
So you are free and encouraged to ask these kinds of questions and even disobey if you find it would be morally wrong to obey them. Just asking questions does not make you scrupulous. It means you are curious and want to learn.
I think previous posters have clarified that if the wedding is performed in the Catholic Church, you are free to attend. Abuses of Canon Law with regards to annulments and remarriage do occur, but you have no direct knowledge of that and so you would in no way be culpable for sin. Even if you know they were fornicating or committing adultery, the culpability would rest with them and the priest marrying them. Plus you wouldn’t know if they went to confession and repented before the ceremony.
If it were someone closer to you committing such sins, such as your own parents, you would be on grounds to take a firmer stand, but you are removed from having any involvement in this couples moral choices.
If they are not being married in the Catholic Church it would be safely assumed they don’t have an annulment:
This article written by Father Francis Hoffman, J.C.D., (serves as Executive Director of the national radio apostolate Relevant Radio) gives a direct answer to your question:
Case 4: “Remarriage of a divorced person without annulment.” Invalid. Potentially remediable through annulment and sanatio (from the Latin for “healing”), which cannot be presumed. For purposes of clarity, an annulment invalidates a marriage, while sanatio validates a marriage. Practicing Catholics should not attend. Possibly against natural law; certainly does not fulfill canon law.
Because of the strong words of Jesus cited above (see Mt 19:9), I don’t see how a practicing Catholic in good conscience can attend a wedding ceremony that he knows will be an invalid marriage. His/her attendance seems to condone what is going on. Rather, he should explain to the individauls that he loves them and prays for them and wants the very best for them, but that he will not be helping them at all if he ignores the clear teachings of Jesus Christ. These can be very hard conversations. But remember what Jesus said: “Everyone who acknowledges me before others I will acknowledge before my heavenly Father” (Mt 10:32). That should be some consolation.
However, some wise and prudent priests with many years of pastoral experience are of the opinion that there could be some cases in which one’s (passive) presence may be legitimate, especially so as not to cut oneself off from one’s relative. For this to happen, the principle of double effect applies: (a) action is good or indifferent (just being there); (b) the intention is good (keeping lines of communication open); (c) the good effect intended is not a consequence of the evil effect; (d) there is a proportionately serious reason. In that case it might be advisable to attend the ceremony and miss the reception, and indicate as well that one does not approve the union.
Only time and the Second Coming will tell if you made the right choice.
With regards to your personal struggle with mortal sin, please, please do not be discouraged or afraid!!!
Everyone struggles and it can be especially hard at your age.
You don’t want to fall into the two traps of the devil:
- Scrupulosity
- Laxity
This can take a long while to figure out. Be patient with yourself. Jesus will be!
He loves the fact that you love Him enough to take up the struggle!
Just turn to him with an Act of Contrition and promise to get to Confession as soon as you can. If you can’t get there until the end of the week, tell Him!
He is your best friend. Don’t be afraid of Him. Be completely honest with Him.
How would you respond to a friend who owes you money and says they’re sorry but they will get it to you at the end of the week? It would depend on how sincere that friend has shown themselves to be, right? So be sincere and talk to Jesus as your very best friend.
Also, keep learning the tools and tricks to avoid and combat mortal sin. Many posters here have knowledge to share. One thing to consider if you are struggling with any kind of addiction is to seek the sacrament of the Anointing for the Sick. It is a very powerful and much underused sacrament!
Jesus will be looking at your heart and your sincere intentions. I am only so glad to know a young person with a heart as pure as yours. Your intentions are very clear for all to see!!!
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