Has Jessie J met Willow Smith?

It's because she has a stage name e.g. Miley Cyrus is Hannah Montana on stage sometimes. The Jessie is obviously short for Jessica and the J means different things its crossed between Jazzhands...

Willow Smith creates the hair style designs herself, often creative and unthinkable. Willow does the hair herself, or occasionally a make up artist will do it for her performances and appearances....

Yes. He is also the father of Jaden Smith. Her father is Will Smith. Married to Jada Pinkett Smith, her mom.

Her brother.

Correct. Willow Smith the daughter of Will Smith.