Jessie was born the youngest of three girls and has two sisters, who are five and seven years older than she is, who were both head girls at school. Unlike her academic sisters, Cornish has stated she was "never really that good at anything". She said, "At school they were like 'oh, you're a Cornish girl' and they kind of expected me to be the same as my sisters. Give me something to draw or an outfit to pick for someone, or hair, make-up, acting, write a song,