I agree with the person who suggested using other colors for the border. Personally, I would not use the green for a border. I would probably use the purple outer border. You might want a thin inner border of the green to separate the purple border from the purple blocks.
As to what to do? Give your sister the quilt. She can return it to you if she doesn't want it but at least you'll never have to lie. If Sharon finds out someday that you have the quilt back, you can say that it didn't fit your sister's color scheme and you were so pleased that she returned it to you because you had a hard time parting with it. Yes, a little over the top, but at least you're not lying.