08/03/2011 at 07:55 PM EDT
More than 9 million viewers tuned in to The Bachelorette
season finale Monday night and watched Ashley Hebert get engaged to the man of her dreams
, New York construction manager J.P. Rosenbaum.
But they also saw Hebert's sister
, Chrystie Corns, loudly voice her disapproval of Rosenbaum as a mate for her sister.
"I made a big mistake," Corns tells PEOPLE of hastily judging Rosenbaum and telling him he wasn't a good match for Ashley. "As I started watching the season, however, it's very clear to me that J.P. was the one. He was the best choice."
On the May afternoon that they all met, Corns was singing a different tune. From the start, she told Hebert and Rosenbaum she didn't see a connection between the two of them, even going as far as telling both that she saw more of a bond between her sister and Brad Womack on The Bachelor
"What I didn't take into consideration at that point was that she had been on this eight-week journey and she was really tired," says Corns, noting that Ashley is usually much more animated, especially around her family.
Corns (who appeared on the first season of TLC's Extreme Couponing
) has since apologized to Hebert and Rosenbaum, whom she now refers to as her sister's "knight in shining armor" for being such a rock all season long. Corns says she and Hebert are "the best of friends." As for Rosenbaum, he isn't letting her off the hook just yet.
"I'm totally apologetic, I feel like a jerk – I want to make it up to him but he won't let me forget it," Corns says with a laugh. "J.P. says if he has anything to do with it I'm going to have to fight for the right to be maid of honor."
For more on the couple's engagement, their relationship challenges and their future plans, pick up the new issue of PEOPLE, on newsstands Friday