Will Seaton, from Indiana, started dating Ashley Schaus in 2010.
She told him that she and her sister Hannah, who has diabetes and Down's syndrome, were a "package deal".
Hannah was invited to the majority of the couple's dates, accordingly.
Seaton asked Ashley to marry him in March.
So it was only natural that Hannah was included in this, being asked to be Will's "best friend forever" at the same time.
Ashley told Fox 5:
It was the sweetest moment in my whole life watching Hannah be so happy and feeling so special
And then he looked at me as I was trying to stop sobbing. I asked ‘Am I next?’ Then he got down on one knee and asked me to be his wife.
Hannah was given her own ring, an heirloom that belonged to her grandmother.
Hannah said she was thrilled to be included in the proposal:
He takes me fishing and makes funny jokes. He makes me laugh and takes care of me.
The wedding is planned for October and Ashley says her younger sister will play a major role in the ceremony.