INFO: (this seems kinda fake because where are these details?)
Does you brother take his meds, and is his viral count zero. If so there's zero chance of him transmitting the virus. If your brother is being cared for by an HIV doc and no longer actively engaging in risky behavior he's probably one of the lowest risk around given he'll be tested quarterly for STIs.
Even given #1, there's now PrEP, a med Melissa can take to prevent getting HIV, even if your brother could transmit. (There are also obviously condoms.)
No one commenting here seems to know the above. HIV is now pretty easily managed.
YTA (& hugely) if your brother is now responsibly managing his HIV, and you jumped in and told Melissa without explaining any of this.