Behind Closed Doors by B.A. Paris

Grace thinks that she has found the perfect match in Jack. He's handsome, wealthy, and is happy to have Grace's younger sister, Millie, who has Down's Syndrome, come and live with them. However, right after their wedding, Grace discovers she has married a monster. Jack delights in imprisoning Grace in their home and controlling what she eats and wears. He is also anxious for Millie to leave the group home she's been living in and move in with them because he will then have two people to torment. Grace knows she has limited time to somehow get away from Jack and save her sister from the hell she's experienced. How will Grace succeed when Jack never leaves her alone? If you can suspend a bit of disbelief for how Grace gets herself into this situation in the first place, you will be treated to a nail biting tale that you will want to hurry to finish.