Hi, I'd like to help you with your question.
I am so sorry to hear about the loss of your brother.
It sounds like you are a wonderful source of comfort to your mother. What you have tried so far is good. However, she may need to express her pain and though it is difficult, that is what she needs. What you can do is be there for her. Talk to her about your brother and allow her to talk about her feelings and do not judge anything she says. It may be harsh, very sad or something else hard to hear, but that is ok. Death affects everyone differently.
Ask her if she wants to plan something to remember your brother. A special event, maybe a tree planting in his honor, doing an activity he loved, visiting with others who grieve him on this day may all help her mourn.
Don't forget to care for yourself as well. This was your brother and although you are trying to help your mom, you need to take time to mourn as well. However you need to express that, make time during the day to do so.
Some books that may help are On Grief and Grieving: Finding the Meaning of Grief Through the Five Stages of Loss by Elisabeth Kubler-Ross and David Kessler and When there Are No Words: Finding Your Way to Cope with Loss and Grief by Charlie Walton. They can be found on Amazon.com or your local library may have them available.
If you feel your mother's grief begins to interfere with her ability to live her life productively, help her seek counseling. You can contact her doctor or pastor for a referral.
There is also a site on line called http://www.goodgriefcenter.com/. They have resources and ideas to help those who are grieving.
I hope this has helped you,