A question: just how many members are there in this "extended Millham clan?"
One of our more famous members, Esther1981, has mentioned similar instances in one of her games. Like yours, almost all of the victims of this infanticide were members of her sim's family that have moved out of the house. And when she posted a screen shot of the family tree, it literally spanned the entire screen lol. Now
that's a big family!

Likewise, Twinbrook is infamous for it's initial baby boom, so it's entirely
possible that the game simply decided at that point that your computer couldn't handle the excess population. Then once the population evened itself out, this terrible effect stopped.
There are two cold and hard facts of life in the Sims 3: infants and toddlers cannot live on their own, and they do not often form relationships with anyone but their parents. It is likely due to these two factors that the game is programmed to target "extra" infants first. It can't kill of their parents as it would leave them living on their own, and it's far more likely that their parents have (or could develop) relationships with active sims.
I can't totally rule out the possibility of a glitch in either case. Since this is an actual game mechanic, it's entirely possible that it was glitched in either or both cases. But if the population of the town is exceptionally large, it's most likely working the way it's supposed to work.