Ashley Schaus, 23, met her fiancé, Will Seaton, in 2010 after they both attended a car show.
Not long after they started dating, Ashley introduced Will to her sister, Hannah, and told him they were a package deal.
"She has Down syndrome and diabetes, and I look after her and protect her," Ashley told FOX 5's Katie Muse.
Ashley said Will went above and beyond to do just that, and has treated Hannah like a little sister from day one.
Ashley said after Hannah's initial reaction of being surprised and happy, her sister then cried.
According to Ashley, her sister will share vows with Will during the wedding ceremony.
Ashley said Hannah, 15, is on cloud nine and has enjoyed the attention of being the "best sister.". »