Hi Claire - I agree with Ahoy, I did not think you said anything you shouldn't have with MIL, you were honest about your situation and how you felt, she cannot find fault with that. And it's good she wants to remain close to you and D.
I am close to half my H's family, the other half have sided with him, met the OW and are entirely happy with his choices. It's just such a difficult situation for in-laws too.
Take care x
Me - 44 Husband - 47 D20, S18 BD - Aug 2013 Moved out - Jan 2014 OW discovered Jan 2014