No problem. My older brother was born with a minor brain injury, made poor decicions in high school, got a girl pregnant and married her, went on welfare, had another child ... and his caseworker offered him a financial incentive if he’d get a vasectomy. My brother thought that was a great deal (he couldn’t really care for his two kids, who ended up being adopted by my parents), so he took the money and had the procedure. He’s fine with it — it allowed him to be less restrained with his sexuality, and he made a few easy dollars. Was my brother, whose brain doesn’t work quite right, “coerced”? Maybe. But we’re all fine with it.
Your older brother is an interesting case. I would say that whether or not his sterilization was ethical would depend on whether he was capable of giving informed consent; i.e., whether or he was capable of understanding what he was consenting to and making the choice rationally.
In the case of the inmates in the story, there is grave doubt as to whether informed consent was given, or even possible. Firstly, some have said that the operation was done against their will. Secondly, others have said they were pressured by prison authorities, which a situation under which autonomy is compromised due to the power disparity. Altogether, a very different situation from that of your brother.