Im rootting against the little sister, All indication is pointing towards her winning (big sister isnt getting any screentime) But she has been nothing But underhanded and mean, she is fighting for her love by denying MC Access to her crush and just Hopes that shaming and proximity Will Dispell MCs crush
I dont like it, i feel sorry for the MC
Eh I don't really view the little sister as that underhanded or mean tbh. She's jealous and isn't good at expressing her feelings, but it's not like she's blackmailing the MC into sex or anything seriously fucked up. I wouldn't even say she's REALLY doing everything you're saying, I believe she's mainly teasing the MC for attention. She's not actually keeping her from the older sister, and she knows that the MC's crush on the older sister isn't going to fade just because of her actions. The way you're describing it sounds like some hardcore NTR to me, which I'm not seeing in this story at all. Especially since this is such an ultimately lighthearted comedy, that's not even meant to be taken that seriously to begin with.
They're also young and have a lot of growing to do, they're teenagers sorting out their feelings and they're going to do some dumb shit, that's youth. As long as the younger sister's character progresses, and she learns that just talking to the MC is a far better method than this silly blackmail, I don't really see any reason to hate her.
Edit: Thinking about it, funnily the MC is honestly the one who has done the most questionable things so far. With the older sister body pillow, wanting to smell the older sister's used towel, so fourth. Again though, it's an absurd comedy so we aren't even meant to take these things seriously. If this were a different kind of series, the MC herself might be some kind of seriously disturbed stalker or possibly Yandere.
While she is obviously teasing a Lot, she is also straight Up blackmailing MC to spend time with her and pushes herself in front of MC everytime MC tries to Hangout with the sister
As for MC, she is just has dirty fantasies which isnt abnormal. a Lot of the things you List about her are cliché crush jokes