Agreed. But what disqualifies her (& the brother!) is to sneakily send out single invitation after agreeing with her parents, and no heads-up visit or personal call beforehand.

This. I'd rather not have a wedding than condone homophobia and let my family treat someone I value like they're inferior.

Yeah, but it's obvious a fancy wedding is more important to the bride than celebrating with people she actually likes. It speaks ill of both the bride and groom that they don't realise how big of a deal it is to exclude the SIL, a close family member, from the wedding because of pure bigotry.

This is yet reason that big weddings suck! Because they're so expensive, beyond the means of most brides and grooms, they have to dance to the tune of whoever is paying, even if it means losing friends and alienating family members. I hope the OP sticks to her guns, because even if the bride feels like the wedding is the only thing in the world that matters, well, that's bullshit. Weddings aren't important, loving friendships and family relationships are, and the bride sacrificed those for the big splashy wedding.

She's only in that bad position because she let herself get into it. As soon as her parents said that she could not invite her sister in law's wife because they were paying for the wedding, all she had to do is say "Ok, you're no longer paying for it, we're having a small ceremony with close family only, including my sister in law's wife".

What mother doesn't help with the wedding dress but plans a big wedding?

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SIL is in a tricky position. Even if she disagrees with her parents, she won't be the first person to tolerate their parent's bigotry because, well, they're their parents. Wedding days are going to be the one day you really want your parents there, if you don't hate them. It's not the time to chip away at their ignorance - she should have done that years ago and she wouldn't be in this position now.