I found this question via Google while looking for an anime with the plot point of a protagonist raised by a robot. The anime I was thinking of was Genesis Survivor Gaiarth.
A 1992 three-episode OVA series, the protagonist is raised by a soldier robot who wants to keep the boy safe from war. But war catches up with them.
This does not have the brother element as near as I can tell, and the time doesn't line up, so I don't think it's what you're looking for, but maybe you misremembered some?
Another series with a child raised by robots is Space Family Carlvinson, though that's much more light-hearted.
