Isn't this the Aki sisters? Or are Chen and Tewi pretending to be such? (How did they hide their ears?)

Nop, they're Chen and Tewi changing into adults so they can buy sake.
Plus is a pre-Imperishable Night doujin so no Aki sisters to you.

See the leaf on their heads? They're using those.

Name of doujin?

Kitten Girl
I think the translated version is in gensokyo.org.