My sister was born legally blind and more, later we found dad was Army Infantry 66-68 Vietnam, so is there possible AO connection?

It could well be there are birth problems of defects that are passed on to the children of Veteran's that were exposed to Agent Orange from the exposed parent. The connection to agent orange is possible but proving it is may well be an uphill battle. That may have happened anyway, its hard to say but I doubt the VA or the government will be much help to anyone on that. In any event it would be a good idea for Your father to have an Agent Orange screening Himself. I had that done by the VA esp as I was in an area of Vietnam where Agent Orange was in very heavy use.

Good Question PO3 Aaron Hassay !
You might want to get the opinion of SGT Robert Pryor SP5 Jim Curry SPC Randy Zimmerman @ PO3 Bob PO3 Bob McCord @ TSgt David TSgt David L. SGT (Join to see)

Here is the VA website that addresses your question:

https://www.publichealth.va.gov/exposures/agentorange/benefits/children-birth-defects.asp

I didn't see blindness on the list, but it'd be worth following up with someone at the VA.