I am really sorry.
This is a difficult time.
But why is his mother taking legal action? I'm not really understanding..
This is just my personal opinion, for whatever it is worth, but if you are divorcing, it generally comes down to just a couple of things. Money and children. If you have no children, that really simplifies it all. Now it's all about cash.
Hire a good divorce lawyer. Let him or her walk you through this. All any of us here can do is just give you some general advise. You need someone by your side that knows exactly your situation.
as an aside, I don't know how your husband pays the bills, but you really should probably get yourself up to speed on what they are and how much. I know, my wife has no idea about ours, but I do them all online, and it's really simple to see it if you know how.
Good luck!