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We know what happened to Touka and Ayato's dad, but what about their mom? We don't hear anything about her at all.
In Tokyo ghoul:re ch. 70-71 it is revealed that Touka's mother (who was also Renji Yomo's sister) was killed by Arima. I believe the exact incident was not shown, but presumably she held off Arima while Arata (Touka's dad) escaped with the kids.
Required, but never shown