WARNING: long response.
You were in a group of 5 long-term friends, but after a falling out which left you feeling extremely hurt 2 of the girls decided just to cut me out of their lives.
Whatever the true cause of of the fall-out, you believed it was motivated by jealousy of your superior lifestyle/career choices on the part of one your friends. You were therefore additonally hurt when another friend who, in your opinion, was a complete sheep, not very bright sided with the jealous one because she was told to.
It seems that you never had much liking or respect for the "sheep" but, as you clearly rated your own intelligence above hers, I suspect it rankled that she didn't follow you.
Five years later you discovered that the "sheep" had her comeuppance at the hands of the allegedly jealous friend. At this point, you should have revelled in the wonders of karma and left well alone. Instead, you invited the "sheep" back into your life.
Did you do this to validate your take on the reasons why your formerly close group split up? Did you expect that the "sheep" would so grateful for your magnaminous forgiveness that she would dumbly transfer her allegiance to you and follow your lead?
If so, you severely miscalculated as the "sheep" gradually became feted by your/dh's friends to a point where she took over one of your best friends and will shortly be married to one of dh's.
The problem is with your ego. The "sheep", who you still consider to be an inferior being to yourself, is now centre stage and part of your hatred may be caused by the fact that others have failed to see her as you do or, more aptly, see 'through' her in the way that you do - but, as they're not you and don't share your experiences with the woman, why should they?
Hatred is one of the more dangerous negative energies; it can boomerang back and bite us on the bum, which is what it seems to have done to you in the form of arguments with your dh.
You're best advised to let go of the irrational hatred you feel for this woman and let the law of attraction work for good in your life. Karma will take care of the "sheep" - and it seems it's already begun to turn the wheel.
Laugh at yourself for indulging in teenage angst and, if you can't wish the woman well, don't wish her ill.
Your first step on the path to a hate free future is to attend her wedding with a big smile on your face, and not let one word of criticism of the bride denigrate you.