The sister got jealous of her and destroyed the field and then she covered for her making the brother think it was her. (just my thoughts) if it turns out like this, it'll be pretty disappointing
It is probably not really possible. Maybe the sister destroyed the field ( yes ik she was 4 years old), because she was angry that her oni-chan wasn't paying attention to her and then the girl took the blame or something.(prob not)
If I were the mc, Imma slap that bitch take my sister from her and say this word to her
"Leave me and my Imouto alone, Don't you dare talk to me again"