This appears to be from an OAD but I can't quite make it out.

「懺・さよなら絶望先生　番外地　上」
Zan Sayonara Zetsubou Sensei Bangaichi Jou?
木津多祢
kitsu_???

I heard that Chiri has a sister who is tottaly her opposite... so Tane is her sister?

You should already know the pun in Chiri's name. (Kitsu Chiri / きつ ちり --> kicchiri / きっちり)

Tane's name works the same way. (Kitsu Tane / きつたね --> kittanee / きったねぇ, which is a spoken colloquial form of kitanai きたない meaning 'unclean, messy', etc.)

I just noticed that there's someone eating her out.

When you mentioned that, the clues all came together. Particularly the presence of one pair of panties on the bed, and another around her ankle.

...Oh yeah--and the razor blade on the bed.

But they still haven't revealed Chiri's Younger sister yet, you know, the one Chiri injected juice into.

Zanz said:
I just noticed that there's someone eating her out.

Looks like it's Chiri herself.