She also revealed that her own mother called her selfish and said she would "look bad" if she didn't bid in the auction.
"I just find this really awkward and tacky. She's basically having people pay to be in her wedding. It's totally nonsensical," added the sister.
The woman's post received hundreds of responses from shocked people online, who dubbed her sister "selfish" and "a nutter".
"In my opinion her sister is highly manipulative," wrote one. "The nutters are those willing to actually pay to be in her wedding party. Why would anyone do this? I can't wrap my brain around it."
One user said the whole affair was "absurd and trashy" and others said they were struggling to get their heads around it.
The post has amassed 500 comments from shocked Reddit users.