How can i get out of going to a wedding?

Just say that something important came up that you absolutely can't get out of. There's no need to explain, and if she does ask tell her that's it's personal matter pertaining a family member. Then wish her luck for her marriage! Also is there a chance that she isn't replying because she's too busy with the wedding? IF she never replied before the whole wedding, then I don't think she's a good enough friend to attend the wedding.