I actually got angry reading your first post. Your sister!?!? I would have more respect for a necrophiliac.
You need to regain control of your life, before you start making decisions like staying or leaving.
From your posts you seem far too passive. I do not mean this as an insult, but has he faced any consequences for betraying you like this?! An apology won't change a serial cheater.
Start listening to all your "girl power" music or whatever you use for motivation.
I think I am the only man that has responded here, but this is what I would do:
1) Kick him out of the bedroom/house and implement "180". Time for action!
2) Go to doctor and check your health.
3) Go to IC and just let it all out. Vent here as well.
4) Tell him that he has to give you the names and contact info of every affair partner. Get proof of the affair too. If he refuses - this means he is not remorseful, and is just sorry he got caught.
5) Go see a lawyer with the info you have. Get the best deal you can and file for divorce. Do not at any point tell him you are filing for divorce. You owe him nothing. You can always change your mind later, and not divorce him. Also find out how to protect yourself financially, and set up your own personal account if you do not have one. Transfer a respectable sum to that account.
6) After he has been served with divorce papers, expose him to both sides of the family. You should not be ashamed, you are the victim here.If they go nuclear on your husband & sister - tough! - they shouldn't have slept together. Do not contact your sister or let her any where near you. If this was me, she would be dead to me (as well as any family member that supports her).
7) Then get in contact with the husbands/boyfriends of the affair partners and inform them as well. If you were in their shoes wouldn't you want to know if your partner was cheating?
Staying together or divorcing is a decision only you can make. However, you need to regain control of your life. He has to face consequences and should be begging you to stay in the marriage not "Oh I'm sorry, lets move on and play with the unicorns". Also - If they slept in your bed, throw it out and tell him to buy a new one.
I'll be back later to check up on your progress, so keep posting here and just vent if you have to. (Remember the above is only what I would do, this is your life.)