Tatsu-nii's mother seems much more scary than Ri-chan's mom based on their actions. Sure, Ri-chan's mom looks scarier because of her way of speaking, but she doesn't pick up her daughter by the shirt for running around.
I'm missing something cause I have no clue what is going on at the end
I think this is just Ri-chan getting scared that Tatsu's mom got really mad at him. I guess she's trying to look away from what she caused because he was running around after her.
If you're talking about this specific extra, then it's not relevant to the main story at all. It's just a flashback when Tatsu and Ri were kids
IMO this actually ties in to chapter 28 when
Ri-chan says Tatsu got scolded by his mom a lot and this is a prime example of what that looked like.