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By Matt Wotus

When planning to propose to his girlfriend, Will Seaton knew there was another person he would need to ask.

When the day arrived, before asking Ashley Schaus to marry him, Will first asked Ashley’s sister Hannah, who has Down Syndrome and diabetes, to be “best friends forever.”

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Ashley and Hannah are inseparable, and when Ashley and Will started dating in 2010, she told him they were a “package deal.” Most of their dates included Hannah, and the three became best friends.

“Hannah reminds me that there’s a lot in life to be thankful for and that we shouldn’t take life for granted,” Seaton told ABC News. He said his relationship with her is a “true blessing.”

Both Hannah and Ashley said yes to their proposals, and Will gave Hannah an heirloom — the girls’ grandmother’s ring — and Ashley an engagement ring.

Will and Ashley, who plan to get married in the fall, built their home behind the Schaus family home in Santa Claus, Indiana, where Hannah lives. The couple said Hannah will play an important role in the wedding, as she will exchange promises with Will to remain best friends and always take care of each other. The pair will also dance to Harry Nilsson’s “Best Friend” at the reception.

“She will be a part of every process,” Ashley said. “I wouldn’t want it any other way. I am blessed to share this day with her.”

Hannah said that she was “very happy” that Will included her in the proposal and that he is “amazing.”

“He takes me fishing and makes funny jokes,” she said. “He makes me laugh and takes care of me.”