Alright, then it comes down to these options -
Do nothing and sit next to somebody horrific at a Wedding, live with the moral guilt of enabling somebody morally questionable, who if has made up such allegations is abhorrent and deserves the full extent of punishment owed to her, but take solace that you're not the judge and this situation isn't your problem
Closer to the date of the wedding, call 101 or Crimestoppers (probably the latter as its anonymous) and tell them on x date, she will be at x place at y time, and live with the guilt of lying to your family and maybe ruining your dad's wedding, but take solace that you probably did the morally right thing.
Honestly mate, the original warrant question doesn't really seem like what this is about. Nobody here can validate your choice; it's a personal decision and you should do what you feel is right. If you're saying you can't live with yourself for not letting the police know, you've basically answered your owner question. We can't tell you what to do.