Not the same. First, Adam Cartwright was referred to on the number of episodes after he left the Bonanza. He was not totally forgotten. Jim Kirk's brother was mentioned before we saw him and was killed off as part of the plot of an episode. He wasn't introduced as a character and then dropped without explanation and then forgotten. That would be Janice Rand. He was a part of Kirk's backstory.
I still maintain that they knew Kirk had a brother. Harve Bennett knew: he watched every episode before he produced the second movie. I'm sure William Shatner would've remembered aspects of his own character. He might not of remembered the guy's name, but I can absolutely imagine he remembered Kirk had a brother at some point. Even David Loughery might've known it. So even if they didn't mention Sam by name, by having Kirk actually elevate Spock on the same level as his late brother actually makes the scene more important and drives home how highly Kirk regards him.
I just don't see why it's so hard to accept that the people involved in the making of a Star Trek movie might actually know something about Star Trek.