Can money be gifted to a disabled son and be overlooked by Medicaid 5 year look back?

I agree. I would get all legal documents that show who is legally responsible for the support of the disabled son and consult with an Elder Law attorney who knows Medicaid laws very well, in your jurisdiction. I would question if it's gifting, if you are legally required to provide financial support. I'd rely on legal guidance to sort through the matter.

My brother is my fathers dependent. My mother passed last year and my parents have supported my brother financially most of his adult life.

Hold on, I'm sure people will be along in a minute who have experience and knowledge of the relevant regulations.

Meanwhile a couple of questions: is your brother actually your father's dependant? Or, failing that, has your father been in the habit of supporting or financially helping out your brother? I wonder if this might make a difference to Medicaid's view of gifts made to him.