I always forgave shows from the '60s with their inconsistencies in continuity. Back then, television producers were concerned with getting an hour's worth of material onto TV screens, with the thought that it might be rerun once or twice, but that no-one was really paying that close attention to every little detail.
So if Kimble had a black-sheep type brother in an early episode, and he just wasn't important in later episodes, he wouldn't be missed. Or one could rationalize that these black-sheep types tend to distance themselves from the family and that perhaps everyone just knew that old Ray was drifting along somewhere out there in America, oblivious to what was happening in his family. There are certainly long stretches of Richard Kimble's life on the road that we were not privy to in the episodes. Much time would have been spent on buses, in terminals, settling in, finding hotels and work, even shopping for clothes and such. During such stretches, a phone call to Donna could have easily gotten him caught up on Ray's whereabouts.
Dates of past events weren't all that critical either - all that mattered was the episode at hand. I'm sure that had the Quinn Martin people realized how long-lived and how analyzed their show would be in future years, they might have taken better care of these little details. As it is, they're a quaint reminder of how television shows were regarded in a past age.
Even into the '70s, shows had these kind of inconsistencies. In M*A*S*H for example, there's a mention of Hawkeye's mom and "Sis" in his letter home. Later on, we find that Hawkeye only has a father back home in Maine. Early on, it's mentioned that the only book his father ever read was THE LAST OF THE MOHICANS and was young Mr. Pierce was known as "Hawkeye", and yet later it's revealed that his father was also a doctor, so it's rather unlikely that that was the ONLY book he'd ever read.