What Do I Say, If Anything.

My mother in law caught me fucking her daughter a few weeks ago. But instead of making a fuss, or walking out, she stood there, put her finger to her lips to shhh me and began touching herself through her skirt.
I carried on fucking my wife from behind and witnessed my mother in law raise her skirt and pull her knickers to one side. As she stared at my cock thrusting in and out of her daughters pussy, I stared at her pussy and instantly wanted to fuck her.
But that's as far as it got, as my wife orgasmed and turned as she shook. Luckily my mother in law moved quickly and I saw her duck back out of the doorway.
Nothing was said when she made it known she was coming in by knocking after about fifteen minutes, and I definitely didn't say anything to my wife.
We've seen my mother in law several times since then and again nothing has been said, either way.
However my wife will soon be taking a four day working trip to Eastern Europe, and she's asked her mother if she'll drop by and check in on me.
Over last week I've received quite a few messages from my mother in law, and in two of them she's sent naked pictures of herself, one of which shows her playing with her beautiful looking pussy. And the message along with it says "I'll certainly be looking after you. Hope you've got the energy".
Now I could have sent a message back telling her it's definitely not happening. But the same night my wife said this "When mum calls round, make sure you're not tired. She'll keep you occupied while im gone".
I didn't react or say anything about what had happened a few weeks ago, but now I'm thinking my wife knows her mother watched us fuck. And I even think they've decided between them, that I'm going to be fucking my mother in law.
I'm stuck whether to say something before my wife goes away, or not.