You need to get as far away as possible. You have a computer, so how about emailing your local Domestic Abuse hotline and see what they can do. Perhaps you should bcc to your sister all emails regarding your home situation. Also send her any replies that you receive. YOU DO NOT WANT HIM TO READ ANY EMAILS REGARDING YOUR FEARS AND WANTS!! Maybe your area has an underground process to move you and the kids to a safe place.....hundreds of miles away.
Everyone...don't assume that she is not telling the truth. Stress can cause one to forget dates, exaggerate, or even make up something. I would rather see that she goes into a shelter and not a grave!