I need help find a manga

So I read this probably 2 or 3 years ago, so everything I say might be a little bit wrong but yeah here’s what I remember but before that to avoid confusion, we’re going to refer to the sisters as sister (1) and sister (2). From what i remember the 2 mains characters are sister (1) of them is oblivious that she as a sister(2) and the other sister(2) that we are going to see is the "main character"
So sister(2) lives inside of sister (1) heads and to say the least sister (2) LOVES sister(1) , she adores her , like sister(1) is so kind precious and everythinh sister(2)
wishs to be able to talk to her (sister 1 )and everything , like she'll do ANYTHING for sister (1) . Ok so this is where this is going to get confusing because of my memories
Sister (1) get bullied or smth or dies idk but from there sister (2) is going crazy bcs who dares to hurts her precious little sister(1) ?? And from here sister (2) is able to control the body of sister(1) and takes her places and seeks revenge for sister (1) and proceed to become the most powerful person in the world or smth like that
I also forgot to mention a lot of things , from what i remember they aren't actually sisters but that's how sister(2) prefer to think they are and toward the end of the manga i think sister(2) gets married to a demon lord or smth but like she only gets married to him bcs he has something that benefit her and too be clear she doesn't care about him at all like she's really doing all of that for her sister(1) , and sister(2) also kills multiple gods i think ? And the art style is very pretty and detailled but also messy ? Anyways i apologize for any grammar mistakes since English is not my first language and i just hope for someone respond to me and thank you