Is there a hard and fast rule for knowing whether it is scandalous to attend someone's marriage for Catholics?

On Catholic call-in-shows and even on this website, there are innumerable questions about the scandal caused by attending the weddings of people who aren't perfect angelic prototypical Catholics. One or both parties can be: 1. divorced 2. not-Catholic 3. fallen-away Catholics 4. didn't even realize one of the parties was baptized Catholic 4. living together 5. have children out of wedlock 6. currently pregnant 7. a known sinner Which of these should disturb a Catholic's conscience enough to prevent them from attending a marriage and how does proximity to the couple (maybe they're cousins that you don't know so well) affect the decision?