I don't know how long they've been calling their grandmothers obaa-chan, but I do know that the current honorific/relational language scheme has been in place since at least the Showa period; it's in manga I've read from the 30s, and it wouldn't be so pervasive in early postwar manga if it weren't already patent throughout the society. And given a) the names and b) that Kami is thinking about her grandmother in relation to herself (i,e, how she addressed her grandmother), I don't buy it at all.