what I unterstood:
big tits-chan married a guy named OKADA
and OKADA is "coincidentally" also her old classmate (the MC) last name
so she named her daughter the same first name as her old classmate (the MC) which is Ryuusei
the little brother accepted too quickly, why would he risk it and send Takemichi to the past again.
He finally got his sister back after many failed attempts, him and takemichi are the ones who suffered the most and he's the best to know how fragile time is and how easily Takemichi can fuck...