There's still the childhood friend, sciencey senpai, and stepbrother flag-

Blood

Wh-Where's my beloved pure and innocent stepbrother...?!

Heheh... How unfortunate for you.

Ah!

Bog Route 2: Tuesday Suspense

They won't be returning any time soon.

He went on a field trip.

And your father is on a business trip.

Those... are some normal, fairly common circumstances you've set up there.