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.