L.C.
It sounds like you're having a hard time finding a stable, respectful footng where you can move effectively, be a leader and mother the way that feels right to you.
It may help you to think of the things your brother does as if it were a total stranger doing it -- what different response would you have if you felt no obligation to this relationship?
Now you might like to consider what obligation you feel in this relationship? Why is he allowed to walk all over you? What requires you to be meek here?
In an old Oprah magazine I came across an interesting idea about honesty and how lying weakens us physically, particularly in not telling other people the truth. You have been lying to your brother about what is actually acceptable to you, probably out of a desire to avoid hurting him. But consider the damage that tactic has done to your relationship with him, your respect for him and your desire to increase the amount of time you spend with him. Now your frustration is coming out in all kinds of inappropriate ways -- depression, nasty emails, self-doubt and recrimination.
It may help to stop thinking in terms of right and wrong (who is right or wrong and whether or not what you did was right or wrong) and start looking at the whole thing through the word 'effective' instead.
Is it effective to email what you think?
Is it effective to say nothing while your son is antagonized?
Is it effective to protect your brother from the effect he's having on your family?
It may help to look inside to find the values that pull you toward living your best life. If courage is a value that energizes you, consider how you could be courageous in this situation. Replace the word 'courage' with any other value (respect, kindness, generous, faithful, wise) and see if those make it easier to see what you need to do in this situation.
I suspect you know what you need to do, but are having a hard time seeing how to do it and still remain you. Clarifying your values may help you there.