Yes.
It is common. To be the scapegoat.
AND they are all playing a game of denial, avoidance, blame,repeat...
The mother going to the brother is probably a burden for them, and she needs to cry about something. HEr son is maybe not okay to cry about to the brother so she cried about you, even to the point of creating fiction.
It is crazy making, and families have twisted dynamics.
None of it is your fault,
I KNOW how bad it sucks to have the inlaws turn on you. I have been there a few times.
You think you have an ally, or a buddy in the war, but then they switcheroo on you.
He learned it somewhere, after all...
I guess it is really hard, but right now, to save yourself the pain, come up with a mantra...until it eases down...
"I can not control what anybody else thinks says or does about me, or in response to my choices. I can only control my thoughts, actions, responses, and choices."