Get a little info.about Sarah. I think you can attend the wedding and congratulate your half-brother so long as Sarah IS indeed female and not a fallen away Catholic, IOW, she knew and practiced the Faith, received Communion, was Confirmed, and walked away knowingly.  If it’s a natural marriage, it’s valid.  If anything “religious” comes up in the ceremony, don’t actively participate.  At least your half brother is making a commitment instead of just shacking up or sleeping around like the majority of pagans.

I think this is pretty good advice. Also Ladislaus' advice (similar to this) is good.

A natural marriage can be attended and witnessed by a Catholic.

If you're obviously attending only for family/social reasons, I really don't see the harm that is done. Even if you're a well-known "notorious" devout Trad Catholic -- they'll know you, and won't invite you to stuff they wouldn't invite a priest to. That's how you know you're living a good Catholic life -- when they leave you out of their wild party invites and activities AFTER the marriage. When you only see them at major family gatherings, due to your lives being too different (and they know it).

Yes, I'm speaking from personal experience here.