Although I totally side with you here, (I say) you can't ask why you weren't invited. I personally wouldn't contribute to the gift, but just let it go otherwise. What a tough one! I usually wear my feelings on my sleeve and I would hate to see that happen on her special day. I would say just have a lot of drinks, but if it were me, a lot of drinks would mean me approaching her and asking WTF??? Wow, I guess if it were me, I would probably need hubby to look after me to be sure I don't say anything I will regret. I wish I could offer you the perfect solution, but it is difficult when you are dealing with feelings.
I take it this is not the type relationship that you can ask her...what you didn't invite me??? Otherwise you would have done so already.
My opinion...no contribution, no shower, suck it up, and attend the wedding. My SIL is young and we go a lot of places together. I always take pictures (I am a photographer), send them to here, along with gifts for the baby and I never hear a word that they have been received, enjoyed, etc. I just do again the next time because I excuse her behavior due to her age. Not even a stupid myspace recognition when she uses the stupid pictures, but I'm not bitter!
To bad your not in Vegas or I'd say let's go have a drink.