If it was me, since it is such a beautiful quilt I would give it to her now. Wrap it in Christmas paper or just add a Christmas bow and say, my love for you and my joy to make you happy is just overflowing so I want to give you my Christmas gift now. Then at Christmas time give her something small made out of same/matching fabric or similar fabric used in the quilt and say this is in addition to the Christmas gift/quilt you received earlier this year. Could be a small basket, pouch, candle holder, pillow/pillow sham, etc. I think Christmas gifts for adults are more about the thought and love we share for each other rather then giving on a specific date as it is for children. However, do whatever you are most comfortable with. Either way your sister is just going to love this beautiful quilt. Thanks for sharing your awesome creation.