I will admit I don't ge the whole "why does it matter who throws a shower" bit being tacky. Times have changed.
I have three children. First child, I had three showers given for me. One by my parents, one by his parents and one by co-workers. Sex unknonw. Second child, sex was a girl. My parents threw me a shower and my co-workers also threw me a shower. Third child, second marriage, sex also a girl. My mom again threw me a shower.
Did I expect or request any of my showers? No. Did I appreciate each and every one of them? You betcha.
Originally, I believe showers were meant to help start a young couple with needed items.
Today, I think it is more of celebrating the birth of a new child. No matter that we may have things from our previous child, why wouldn't we celebrate the life of an additional child? Of course, we do not always need the big items any longer, but one can always use diapers, bibs, spit up clothes, onesies, etc. Sometimes, no matter how hard we try, that darn spit up never completely comes out of those clothes.
I now have a grandchild of my own. She is five. She will probably be the only one for a while due to ages and circumstances of my three children, but no matter what, I intend to celebrate each and every one I might have in the future also.
If those I might invite to a shower choose not to come, that is fine. That is their choice for whatever reason. I am not going to judge anyone over their morals or beliefs.
As to your situation, this is your brother's first child. This is your mother's first grandchild from her son. She is estactic. I don't see evidence of a gift grab either. You don't have to get anything big or outrageous. Diapers and onesies are a perfect gift for any newborn babe.