Groom Exchanges Vows With Bride's Sister, Who Has Down Syndrome: 'I Promise to Look Out for You'

Months ago, groom Will Seaton, 25, also proposed to his now-wife's younger sister.

An Indiana groom exchanged vows with both his bride and his sister-in-law, who has Down syndrome, on his wedding day, just months after proposing to the pair.

Read: Man Proposes to Girlfriend and Her Sister, Who Has Down Syndrome

“Hannah, I’m thankful you’re my sister and my best friend,” Will Seaton, 25, of Santa Claus proclaimed, in a video licensed by SWNS. “You’re one in a million. I promise to always take you fishing, tell you plenty of funny jokes and most importantly, I promise to always look out for you and take care of you as a big brother."

The vows he exchanged with his sister-in-law Hannah Schaus, 15, ended with a hug as they were declared “best friends forever.”

“She was on the verge of crying, I had to help her read [her vows] but it was very emotional,” bride Ashley Seaton, 23, told “I’m really glad she got to be part of us that day and not just part of the bridal party.”

After Will proposed to Hannah and asked her to be his “best friend forever” before asking Ashley to marry him in July, the doting older sister wanted to make sure her younger sibling had a big part in the wedding.

Hannah even got to sport her own white dress on her sister’s big day and her own “first dance” with Will to “You’ve Got a Friend in Me” by Michael Buble.

“She’s was more than a bridesmaid, but not a bride,” Ashley explained. “Will and I have a very good friendship and that’s the foundation of our relationship — [Hannah] just makes the foundation stronger.”

Read: Teen With Down Syndrome Cries as Boyfriend Asks Her to Homecoming Dance

Ashley said she has always been prepared to become her younger sister’s legal guardian when their parents will be unable to take care of her. Sharing her wedding day was the first step in proving she is ready for the responsibility.

“[Hannah] is someone who is very dependent,” Ashley explained. “I’m thankful that I can trust [Will] to take care of Hannah in a heartbeat. He’s one in a million.”

Watch: Couple With Down Syndrome to Celebrate 22nd Anniversary: 'I've Never Seen Love Like It'