Short of starting over there is not much you can do. Give it to your sister, she may like it. If she doesn't like it take it back and buy her something else. If your boyfriends mother asks why you have it, tell her the truth, your sister didn't like the colors but you did. Don't start a relationship with a future mother-in-law with a lie.
The quilt is beautiful and like others have said once the border and quilting are done the look will totally change.