NM,

That was so wrong of your SO to have her daughter lie to her mom. How could the ow condone it?

I don't have any good advice. I don't know what to tell you to with him.

I just wanted to write to you. I'm so sorry that he did that. I know that it makes things very confusing. You are a strong, capable, loving person, you don't deserve to be lied to. What was the, you don't get it do you, there is no other woman line all about?
This is very confusing, even from an outsiders perspective.

I guess it's time to lay down some groud rules and possibly just detach completely and not try to work on this R anymore. Work on yourself, yes, but lay down some guidelines and let SO know that you can not function with the deceit.
I'm so sorry for how hurt and tricked you must feel. I guess the story of the scorpion going across the river is true.
You do have so much strength and perseverance and sound like a really great mother. You really deserve much better.