This is a very dangerous and hard to do solution. Ask your future mother-in-love for your scrap fabrics. The colors that your sister will not like and that will keep her from using the quilt can be recolored if your are super careful and have some experience with coloring fabric. You will need to experiment on the scraps first. Find the product that works for you. I would recommend you check out the ones available from Dick Blick. When you find the product that produces the color you want, sew a small quilt section, top, batting middle and back complete with some quilting stitches. Practice coloring the fabric until you can color only the top fabric with no bleeding to adjoining fabric and no bleeding to the bottom fabric. This is very difficult, but it can be done. Considering the patience needed you probably need to have at least a week to accomplish this after you decide on the product.
There is a good chance of a ruined quilt if you have less than a perfect result. This is usually only done by someone with considerable painting experience or fiber art experience. I recommend you really think long and hard if you haven't done something similar to this before. It is quite hard.