I know how very hard that is, when others just get pg and we have such a tough time of it. You mentioned that another child may not yet be ruled out, if you go this route then I see no reason why you couldn't name your son after your brother. You're not that close to this cousin and he was your brother. Also, lots of families use the same name more than once - DH's family name to carry is John and my family is Mary. There are 2 Johns and DS's middle name is John. I can't tell you how many Mary's we have - it's my middle name too
I am so very sorry to read that you lost your brother .... such sorrow. I think it's a great honor to your brother that your DD carries his name in a different form.
Take care - gentle hugs ... know that we're here -- always.