Should we tell Mom that my brother has died?

Doxrus, I've not read the responses yet; but I've been taught to ask myself when considering dilemmas of this type: "How will I feel about making this decision after ... [in your case, 'after mom dies']"? Pondering and meditating on that question for a while, before I commit to a decision, has helped me greatly. It's not about right or wrong, but about your own peace of mind. Bless you!

I would tell her once and then after nope. You told her she grieved and then you have moved on. I'm sorry for your loss.

Has he visited recently?
Has she asked about him?
If the answer to both of these is no then I would not tell her. It will upset her and there is enough stress and upset not being able to visit "normally" with family and friends.
If she has asked about him and he had been a recent visitor then tell her ONE TIME. After the one time it is "He is at the store" "he went on vacation" he is on his way but the traffic is bad" you can make up any excuse that will work for that moment.