Deathscythe Cutie

The strongest elementary schoolgirl in the underworld has arrived! The Grim Reaper carries a school bag. Residents of the underworld call her "Deathscythe Cutie". Satoko Satou is a no-good woman who's close to having her marriage scam found out and has been caught embezzling money from a yakuza front company. She begs forgiveness at the gang office, and is told that in return for clearing her debts, she is to become the older sister of Lilja, an elementary school student whose true identity is unknown. And so begins the chaotic daily life of the mismatched "sisters"...

(Source: Yawaraka Spirits, translated)