She's acting out, she's lonely, she doesn't feel loved...

Put yourself in her shoes, she has no friends at school, she doesn't even understand them completely or be understood. At home she has to fight for parents attention with her brother... Give her patience, feel for her... Weak kids in her situation will become quiet and timid, strong kids might want to fight with everyone. It's totally understandable. Love her and encourage her, it will get better once she start to make friends