Once upon a time, Alexander was her only son, so it was accurate (at the time). He's also the one that's 'pure' highborn, what with being the scion of houses Rippon and Hart (along with his sister, but he's the eldest, so it's a special responsibility).
Cameron came along and got some screen time in Range War, in issue 16, and his position in the family is explained a bit more there. As the son of Brandon O'Leary and the Duchess Olivia, Cam was never quite accepted by the snooty courtier-types on Vendrell, sadly including his elder (half) brother. So as he was getting a little older, and trying to manage Alpheratz and Vendrell alike, Brandon kind of handed the reins of the Outworlds Alliance/Alpheratz side of things over to Cameron. Cameron then chose to take the O'Leary name (so as not to offend the Lyran nobility, though surely some of them decided to take offense to that decision), as he stepped in as basically a Chief Operations Officer for Mountain Wolf BattleMechs, while his father remained the Chief Executive Officer (and handled the Lyran side of things, the politicking, etc).
...which is also, OOC-ly, my way to keep the O'Leary name (and noble house) alive, setting them up as cousins, and something of a cadet branch, to House Rippon-Hart. Two branches of the family, largely acting independently, overseeing Mountain Wolf's two very different, and very distant, facilities.