I am so sorry for the loss of your brother and I thank him for his bravery and your whole family for the sacrifice. There are simply no words for this sort of bravery and valor.
Maybe you could talk with your cousin? Who knows what his response will be, but nothing ventured nothing gained? As it stands now, I am sure he is only only imagining this part of your post: "While I am honored that they are honoring him this way." The piece about how devastating it is to you not to be able to use the name yourself is not something that someone who hasn't experienced the inability to have children would get at all.
So, maybe if you explained it to him, well, you might be surprised at his reply. Whether they change the name or not, you might feel more at peace with the process. I mean, I guess you need to come to terms yourself with your conflicting feelings--wanting your brother to be honored and remembered vs. not wanting someone else to use the name or another blow to your fertility journey. Even if you decide that the honoring peace is more important to you, you might feel better if you talked about the conflict with your cousin. Just getting things out in the air, even if (especially if)not expecting or getting an action in return, can be so healthy.
Your cousins are not doing anything wrong. They are doing something loving. Sometimes loving things still hurt. I think this is a conversation you can share with them.
PS if you do have a son some day, you can use the name, too. it is okay for families to have two kids with the same name, really, really!! your brother will be twice honored, really sealing his place of importance in your family.
me:smc (single mom by choice)
Dd: Conceived when I was 42 after 2 years ttc. Conceived on 6th IVF cycle after 2 bfn's and 3 m/cs.