Oh boy, been there, done that. As many have said there isn't much you can do but smile and either keep it yourself or give it to your sister as planned. But here's something to consider: you have no idea how wonderful it may turn out once it's quilted. I never really realized how much the quilting adds to a quilt until I saw the "before and after" hanging on my LQS wall. A friend and I were making the same quilt but with a different color palate. She finished her's before I did (still working on it-lots of diamonds, taking it slow) and the shop owner asked if she could hang it up on the wall to advertise the book. I didn't like it at all-personal preference. Then two weeks later I saw it up on the wall again but this time it had been long arm quilted and binding on. It was fabulous! The quilting truly made that quilt a thing of beauty. So wait and see. You may find all turns out alright in the end. You may fall in love with it and hate to give it to your sister. Been there too.
Heidi