A while back I found this manga about girls fighting each other using bug powers MC sister was killed I think anyone know this Manga ?
No I do know that MC is a girl and she was trained young to get the abilities and I think she's an assassin definitely not Arachnid.
- Bug girls - MC is a girl - MC trained young to get the ability - MC's an assassin This seem like Arachnid to me until - MC got a sister that was killed. The only thing different from what you point out.
Well, there's also Caterpillar (same author and theme with Arachnid). There's really not that many insect-mc mangas out there... can barely remember a few others that even use insects as main.