If you do the embroidery, I don't think you'd want the stitches to show on the back. Usually, embroidery would be done on the blocks before they're assembled. I agree that it shouldn't be done at all without checking with the owner, and also sounding out how much she's willing to pay you before you do any of it. She might be thinking she's going to save money by recruiting you.
Ooh, boy - I hope none of those other siblings are inclined to be jealous that you're doing all this work for only one of them! It will be a labor of love, and she'd better appreciate it.