Look, you've clearly thought this through but is it not obvious that this is just something there will be a difference of opinion on? There's no hard and fast rule and typing it like it is an absolute doesn't make it one.

I would never, in a million years, give a +1 to a SO of 5 months. As much as OP might spend, I will spend many times more making sure my wedding is exactly how I want it. Someone I don't know who has been in my sister's life 5 months will not have the opportunity to fuck it up, period.

You can go on all day how gross and tasteless it is, and I'll still think you're just as ridiculous. And wedding websites will be split 50/50 between us on who is right.

Maybe OP should figure out which is the case for this situation specifically by just talking to her sister openly. And also, unless you can tell the fucking future, having an opinion on wedding etiquette does not mean you have any clue if the sister is being homophobic or anything.