You could go see her and have as pleasant of a visit as possible. If she brings up your brother or asks your advice or opinion on anything (other than the weather), you could say something along the lines of :
"Mom....I'm not a JW anymore of course but I do still fully understand what that means in terms of you being able to fully trust anything a non JW like me, has to say about anything important. That being said, I'm going to refrain from making any comments or offering any advice other than to say that If you have concerns about what's going on with your personal affairs, why not have the elders check into it? That way you and I can have a nice visit today?"
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Sometimes the less said the better. Not spilling your guts about your brother when given the perfect opportunity to do so, may garner her trust and spark further curiosity. She may look into the matter and find out what you already know and you'll come out smelling like a rose or at least won't be blamed for stirring the pot.