I posted The Mourner's Bill of Rights and PLEASE on the Child Loss Board. Feel free to print them out as I know it will be helpful to your Mom (they were for me) and all around her trying to support her through this horrific time in her life, losing her child.
She's blessed to have you. You're both blessed. You always have a kind word for everyone and I see you always reaching out to others in pain, while carrying the burden of your own.
One thing I would suggest as you try to help support your Mom is to always talk about your brother. Say his name. Even if it's just in passing, as in walking by her. I had so many people tell me after my Jeff died, and after my asking them why they never mentioned him and expressing to them how badly it hurt me, they would always reply, "We didn't want to upset you." Well, too late for that, I'm already extremely upset!!! And, even more so because everyone stopped talking about him. That was a pain that was also....indescribable.
Another is remembering, especially the little things. Of course, the Birthdays and Holidays will be very difficult and just letting her know that it's OK to not want to do 'anything' if she doesn't want to, will help to ease her anxiety, a bit. These 'firsts' coming up are going to be very painful and she'll need 'someone on her side', so to speak, again, to let her know it's OK to feel anything she is feeling and that you have no expectations of her performing any past rituals, such as family gatherings and/or appearances she attended in the past.
It's going to take a very long time for her just to come to terms with the truth that her son is really gone. Love and Patience. A lot of Love and Patience. And, let her know, let her hear your pain, also. This is such a trying time for both of you and you will both stay in my thoughts and prayers.
You may want to suggest a support group where others truly understand her pain. When she's ready, let her know that 'we' are here for her, always. And, please send her my love.
You are really an awesome daughter.