My parents separated when I was 12 and divorced when I was 16.  My father was unfaithful and there were epic fights but there wad never any abuse towards my mother, younger brother, or myself.  I'm now 33.  My brother odds 32, married, and had 2 kids.

When my brother was 17 he and my father had a falling out and neither of them had ever shared the details with anyone else.   They have been estranged ever since.   At the time my father was using drugs and having sex with questionable partners...it wouldn't be a stretch to say he was to blame. 

I maintain a distant but caring relationship with my father as I love him,  but am unable to maintain a closer relationship (he can be an emotional and financial vampire).  Further,  I have made it clear to my family that I will not take sides. 

After the birth of my brother's first child my mother began developing a growing fear or paranoia of my father.  While she has never discussed it with me, I suspect that she was abused as a child and feel that these experiences are coloring her opinions of my father now. 

This evening my mother overheard me on the phone with my father I  which he asked how the kids were and I responded briefly.  I have always honored my brother's wishes about the kids not having a relationship with him and in no way seem to undermine him.  

My mother got very upset and said point blank that I need to chose sides.  I pointed out that I have honored my brother's wishes regarding his children and that he doesn't even know where any of us live.   She remained angry with me and accused me of endangering the kids. 

I'm at a loss as to the cause or the solution.  My father had never threatened the kids nor asked me to violate my brothers trust yet my mother believes that his enquiry as to their well-being is threatening. 

They have been divorced for about 20 years and she never had any qualms about my and my brothers safety with him before or after their separation.