Demographic is shounen/seinen, set in ancient (fantasy) japan?
A pretty recent manga (2014+), with a classic art style

• Main character (boy) lives in a village with his parents and siblings.
• There's a saying to not wander at night because the wolves/vampire beasts? come down and will kill you.
• MC goes to work, to collect some woods?
• Comes back to find his family dead except for his younger sister.
• Sister is bit, half infected, and strong as hell.
• She can't speak anymore, only hisses and grumbles
• Seems like she only has a tiny bit of human consciousness left
• MC goes on an adventure to find a cure.
  - He has to hide his sister's existence by carrying her in a box on his back.*
• Also tries so hard to not make her sister lose to her desire to eat human.

*****I think this is a unique thing that will make those who have read it recognize what I'm looking for.

Kimetsu no Yaiba sounds a lot like what you're looking for.

What the hell... I thought it was maybe something-yaiba but all I got was this ol yaiba lol. Thank you so much!!