I am so sorry that you are having a lousy weekend. I can absolutely understand how your stress level is peaking.
I am going to try my hand at some advice here but take what you need and toss the rest, OK? I am so aware that we don't get the full picture here.
I agree with Snodderly's last post with regard to property, etc. if anything he requests is family property, that is for your lawyer to handle especially if he really starts to get nasty. For example, get him to send the list directly to your lawyer.
Now, WR it is time to try to stop vibrating. Being in a dark room with soothing music playing very very softly worked for me. If your thoughts run to your situation then let them. The more you try to force them away the more (in my experience anyway) they are harder to chase away. Try to look at things in a calmer way. You are in full reaction mode right now which will not serve you well.
Your first concern is your home. Would that be the kind of message that your parents would give you through your sister? Should you talk to them directly? If they promised they would co-sign and have changed their minds you deserve to hear it from them. Small steps. Confirm this information. I believe you mentioned before that you would not be able to keep your house without co-signers so they should realize how important to you this decision is.
Once you get your answer, then take a deep breath and get out your favourite pen and writing pad and list all of your options. Even if the answer is yes it may not be a practical thing to stay in your house for financial reasons. I can sympathize with how very difficult it must be to face not only this stranger but the potential loss of your home. But like it or not the business side of things must be taken care of by you.
Stop reacting to him. Always wait at least three hours or longer before replying. Come here to post your potential replies. Not because you are insecure but because posting here can help you calm down and stop reacting. Be in control. He is trying to take that from you so he is trying to force you to react. Don't react. I know that is easier said than done but it is worth it for your own sake. Be in control.
Lastly, I can recall BRNR who only posts once in a while now going through some difficult times with relating to her kids. They were both a little younger than yours. Golfmom gave her some really good guidance. You might find reading her last threads helpful.
I hope that helped, if nothing more than to know that someone is on your side and you are not alone in this.
