If I read things right, what we've got here are two 'single-parent & child' families who might one day 'unite' (know-what-I-mean) ... opinions may vary as to whether the parents or their children will make the union come to pass, first.
As for Rei-kun, "It's Good To Be The Ki ... er, Little Brother (a.k.a. Lucky Bastard)". And a (*cough-cough*) "assertive" Big Sister (perhaps in name only) is fine, too - good job, Mitsuha-chan.
Kinda hard to tell if she the daughter of the mans girlfriend in the story. Like how the boy came with his mom however only the girls side wasn’t exactly to clear O.o
His expression right after is the same I'll make when a sequel is announced! (ノ´ヮ´)ノ*:・゚✧