Familicide is *way* overpowered.
1. It's an Epic spell. They tend to be very overpowered. The standard ones in the ELH do things like create a whole new species, produce a silenced, still & quickened Death effect that works on anything with less than 160 HD, grant permanent immunity to any a litany of effects (entangle, hold, imprisonment, paralysis, petrification, sleep, slow, stunning, temporal stasis & web), rain fire & destruction over several square miles for a minute, create a solar eclipse over a 5 mile radius, or do 305d6 damage to one target (save for half). Low end Epic spells ones do effects like create permanent new islands on material plane bodies of water or do 20d6 damage to any creature grappling you (as a silenced, still quickened effect with no material components or XP cost). Epic spells are generally plot devices unless the game itself is Epic level, because they provide a way within the 3.x rules framework for spells of arbitrarily high power.
2. It was (effectively) cast by the single most powerful Evil spellcaster in the history of the world (whose soul is in the possession of the Lower planes, which leaves out anybody annihilated by the Snarl or still alive/undead). She could have lived 1000 years ago, and never got to cast the spell in life (or cast it once, and it rebounded on her as she didn't know the target was distantly related).
These two factors mean that I can buy Familicide as being incredibly overpowered. Even Tiamat herself was furious over the casting, and if Haerta ever cast it in life, presumably she could have brought down divine wrath.