My mum in law has been adding a lot of fire to fights I have with my husband (away from the kids of course). Now our not-so-serious fights have seemed to take a more serious turn. My husband mentioned getting a divorce. Should I talk to my mum in law and tell her (nicely) to stay out of our problems? Or just throw in the towel?

Instead of talking to your MIL about this yourself, I think it will be easier to get your husband to talk to your MIL to stay away from your problems. Also, try to discuss your problems with husband behind the closed door. The root problem is between you and husband. Try to sit down and talk when everyone has cooled down to avoid further arguments. A marriage takes 2 parties to maintain so both of you have to find a way to improve situation instead of getting a way out through divorce.