In a presidential campaign, nothing is sacred. Wannabee first ladies, first children and first cats would be well advised to realize this and start rolling with the punches, because they will come.
They already have.
Chelsea Clinton cried foul when people asked on the campaign trail her about her father's infidelities affecting Hillary - a justified question posed in a reasonable arena. Chelsea, for the most part, got away with rebuffing the questioner, but using that question as a method of attack would have returned if Hillary had gained the nomination.
Barack Obama said his wife was off limits to political attacks, but I don't think those who seek his failure in November are going to heed his words. Likewise, Cindy McCain recently decried the 2000 smear campaign against her adopted daughter, which made some voters think the girl was McCain's illegitimate daughter.
All these people know Washington, and more importantly they know what power does to people. If attacking Michelle Obama on a particular issue means the other guy has a better chance of winning, it will happen.
And if Socks the cat has to die in the process, so be it.