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Your mom's triangulating, trying to either pull you in on her behalf and against your brother or trying perhaps to entice you to speak to your brother on her behalf behind the scenes. You might find it helpful to read "The Dance of Anger"--I forget the author's name--which deals with such issues.
My mother used to pull this routine on me and my brother all the time...til he and I spoke and we mutually agreed not to play Mom's game. If she told him something negative about me, he'd advise her to speak with me directly...and wouldn't run off and tell me. I did the same for him. It's actually worked out very well. Mom is less inclined to do this these days--and my brother and I have kept our boundaries and communciations healthy.
Also, my brother and I have what can be a brutually frank relationship--we always know where we stand with each other. In the end, we're each others' biggest supporters, perhaps outside of our respective spouses. No, we don't come from a highly confrontational background. We just resolved years back to be more loving and healthier toward each other than our mom is with her brother or was with our dad.