As a mother, I don't feel that you should have to cover for your brother. As you said, she raised him, she knows him. I have three children and I know them all and how they are.
My son, (Oldest child who will be 29 this Sept.), is like this. It was not until recently that he brought me flowers for just so. So, maybe he is finally getting a clue. Now, he will call me and wish me a Happy Birthday or Mother's day, but he does not think to get birthday presents or Christmas presents. He did give me something small this past year. I really don't care about myself, I know how he is, but he has a niece and my mom, his mom mom, has done a lot for him. He could at least get a small token. This past Christmas, I did not want him to show up empty handed, so I got some small gifts, wrapped them and had him sign the tags. I think that might have been the thing to give him a clue. I do realize he is short on funds also, and I don't expect much. The phone call is enough for me and when he does something more, I am pleasantly surprised. (Just so you all know, I am not about gifts, it does not really bother me if he gets me something or not, but I do like when he calls me).
I do have some questions...do you ever ask your brother about going in on the flowers for mom? or do you just sign the card? Does he live close enough by that you see him to ask him about going in on the flowers and making sure you get your share? Or is this a distance thing? Does he know you do this every year?
I think it is time to call him and discuss this with him.