Essentially, if your sister has not formally renounced the Catholic faith (a most grave sin in and of itself), she is a baptized Catholic trying to marry without the Church’s approval. As such, you are not permitted to attend the wedding because it would be an endorsement of a non-sacramental marriage (if God does not show up, neither should you).
In order to formally defect from the Church, she must receive, in writing, notice from her bishop or pastor that he acknowledges she has so defected (this is a new clarification to the law; it used to be less strict).
You can read more about the issue here[/url:2w3trw0a], in a blog post by Jimmy Akin.
(It should also be noted that I am very much not a people person.)