Didn't Nichol Simson's and the other victims parents sue OJ?
IIRC, California law allows the executor of one's estate to sue on behalf of the estate - in that case, the executors were the parents, as both were unmarried at the time. Even if Florida law makes the same allowance, Michael Schiavo is still the executor of his wife's estate, and still the only one with standing to sue. The laws vary somewhat from state to state, but I don't know of any way that her parents can do it.