The way I see it she's going to find out sooner or later. I'd be for being honest about it. Besides, it will be a relief for YOU. To continue hiding it is burdensome, that which YOU don't need. We'd all just like to *get over it*, wouldn't we?
Also consider, though, that for your husband, it probably feels to HIM, that YOUR condition, is somehow a reflection on him. In other words, his Mom may be thinking, "How did you let this happen? And why haven't you fixed it yet?" OR maybe this is what HE thinks she will think...
Just a thought....let us know how it goes ;)