M.S.
Do what's right...
As much as you disapprove, this is your brother's choice. He's the one who will have to live with her after it's all said and done. I'm assuming that you have voiced your concerns to him directly? After doing that, it has to be left up to him.
You seem very concerned that your brother is not following the man made doctrines set forth by your church. Please remember that a marriage is still a marriage, even if it's not in a church. If they have any kind of relationship with Christ, they will recognize the covenant they are about to enter. If they don't... again, that will be their issue to work out with God.
Re: your commitment. Regardless of WHY you said "yes" to being in the wedding, you said it. You are obligated to keep your word. (God says, "let your yes be yes and your no be no.") In hind sight, you probably should have said, "Let me think about it." Then, told her you wouldn't feel comfortable because your older than them, etc... but are honored to have been asked. Quit looking for excuses/lies to get out of this and honor your brother, who you love on some level even though you're not close to him.
Backing out, causing his bride to get upset, changing their wedding plans, etc. will only cause dissention or hurt feelings. Look at the long range picture here. Family is precious and is a gift from God. They will stand by us when the rest of the world turns their back on us. Build that relationship with your brother and his fiance'. Who knows, maybe God put you in their lives for a time such as this.
Be blessed and be a blessing,
M.