Honestly, I don't think that we have enough information about the Mother, specifics. We have generalizations, and a few sound bites. I am not even sure that "Mommy Blaming" is even appropriate. A lot of MIL's have a contentious relationship with DIL's, but few husbands actually go as far as family annihilation.
CW's Mother probably reacted as most women would act, denial that her son could do such an atrocious act. Then, projection on the victim, to depersonalize her (SW), ignore the children, focus on the "witch" that drove her precious son into a killer. The Mother has ignored the affairs, or acknowledges them, justifying her son's behavior.
Even a Mother in complete denial, can't overlook killing children, and dumping the bodies in oil tanks. She can hate on SW from here to forever, but CW killed those babies. No fit of rage there. No act of passion. That was calculated and deliberate.
I don't think there is enough back story to really develop a comprehensive analysis. But definitely, it seems like an unhealthy dynamic. And, ultimately, CW killed his family. He made the decision, as an adult. We can't blame Mama for that...