Katie Musser shares an unusual bond with Casey Bender, a deep friendship that started against the odds.

Still, Musser took everyone by surprise at her wedding when she paid tribute to Bender, her groom’s ex-girlfriend and the mother of Musser's new 4-year-old stepson, Landon.

The bride paused while saying her vows to ask Bender and her husband to stand up among her guests, and then proceed to thank them for their friendship and allowing her to be a part of Landon’s life.

“I promise to be an amazing mother to your son. I will guide, teach, and love him every single day,” she said, before promising to “respect, work, listen and communicate” with them as co-parents. "We are one family, always.”

Musser then continued her vows to Landon, after calling him to her side and expressing her excitement at officially becoming his "bonus mommy."

Bender, 25, said the moment completely took her by surprise.

“Just being invited to the wedding was enough for me, but for her to take even three minutes out of her once-in-a-lifetime opportunity, the one day where it’s all about her and her groom, was such an amazing, selfless thing," she told TODAY. "I was just floored. She did not have to do it.”

Casey Bender gives bride Katie Musser a hug during her reception. Katie H. Muss/Facebook

The two women did not always get along this well.

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Musser first met her now-husband, Jeremy, in college shortly before he became a father but was no longer in a relationship with Bender.

“I was very upset and very hurt at the time,” Bender recalled.

Rumors flew and tempers flared on all sides.

Eventually, Musser, now 24, reached out to Bender shortly before Landon’s first birthday. Bender wasn’t immediately ready to accept the gesture, but the two ultimately talked. Their discussions initially centered on Landon, but slowly their conversations drifted to other topics.

“We bonded on so many more levels than just Landon. And we realized we had more in common than we didn’t,” Bender said. “At the time, I thought we would be able to be cordial and co-parent. Never in a million years did I think she and I would be the very best of friends.”

Katie and Jeremy Musser with Landon. Courtesy of Katie Musser

In 2015, Bender got married to her husband, Tyler, who also helped raise Landon. The two Pennsylvania couples live about three hours apart, but coordinate visitation schedules primarily through the women.

As Musser prepared for her Sept. 23 wedding, she struggled for hours trying to write her vows.

“Then I realized I’m missing something, the piece that helped shape me and Jeremy’s relationship and helped us both grow up,” she said. “I needed Casey to know how thankful I am to her."

She folded in promises to Landon, as well as his mother and stepfather, into her wedding vows but didn’t tell anyone about her plans, not even the groom.

After Musser singled out Bender and her husband during the ceremony, she then called up Landon and crouched down to recite her vows to him.

"I promise to stand by you, listen to you and love you every day," she said. "I promise to love Daddy with all my heart. I promise to be a best friend to Mommy and your Daddy Tyler. We will be one big family that I promise you will have for the rest of your life."

Landon, 4, is held by his two fathers, who stand next to his two mothers. E. Heckerman Expressions

Musser posted video and photos from her wedding last week on her Facebook page, recounting the journey the family has taken. She did so hoping to inspire others to co-parent thoughtfully.

“This can happen. We want people to see that the kid is what matters, his feelings and the love he gets," she said. "Obviously, we got a good friendship out of it, but in the end, we really wanted what’s best for him.”

Bender agreed, adding that being best friends isn't a requirement be good co-parents.

"You just all have to agree the child's interests are what matters," she said. “I know at the end of the day that Katie loves Landon just as much as I do and as a mother, you can’t ask for anything more.”

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