I love your quilt layout!
At 90 x 110", you have 15" of overhang on all sides on a queen, and an extra 15" for the pillow tuck. On a double bed, the overhang will be 18" on each side and the foot, with 17" for a pillow tuck.
I would ask your sister if she likes a pillow tuck - my family don't, so never add that extra length because they don't want it, and the bed looks funny if they just let that much extra length hang at the foot. On a double bed, that could end up touching the floor at the foot.