I just went and read the original reddit, and I have to say, this guy has a rather creepy relationship with his sister that makes me wonder...

No I agree, the whole bride being controlling is a red flag, and he should get to chose his wedding party, BUT:

Apparently one reason the finance didn't get along with her is because she walked in on him cuddling with his sister. Now as a child, I admit I have cuddled with my siblings but their ages are 24 and 18, that's a little odd.

Furthermore, at the end of the post he talks about being excited that his sister is going to move in with him now that his fiancé has left. I know some people live with they brothers, but I guess it's the same apartment. Is it a one bedroom? He wouldn't have needed one if it was just him and fiancé, and their young, probably not ruck with a big apartment.

Also, he sort of explains that his sister would never get the wedding of her dreams, so he talks about why that's so important she's there, almost as if she is living out her wedding through him.

I mean I can see how these maybe reflags for the bride.