My sympathies for your situation - it's difficult to have to see such terrible diseases take away those we love.

That being said, are all individuals in a position to take preventative measures against HIV? And does research of a disease (and ways to treat it) only benefit development of drugs that treat that disease?

This is a very US-centric view. Further, it could be applied all the way down the rabbit-hole. "My brother has MS, and my sister is a few years from a diagnosis. It's nice to know money is being spent on dementia treatments, a disease that almost exclusively affects the elderly already near end-of-life, when that money could be going toward researching cures for diseases affecting people with much more of their lives to live."

Effective altruism sounds nice and fuzzy, but it's pretty silly in practice. It's also not like we can just point all scientists and scientific organizations toward a few of the most highly problematic diseases. At best, there'd be an enormous amount of resources wasted on duplication of efforts.