Does anyone know how many siblings has Hermione got? Are they wizards/witches too? Are they as clever as Hermione?

Hermione doesn't have any siblings. However, if i remember correctly, J. K. Rowling once mentioned in and interveiw that she almost gave Hermione a little sister.

Hermione, I think, is an only child.

Hermione is an only child. She is a Muggleborn so if she did have any siblings then they would probably be muffins.

Hermione's a only child, although J.K Rowling did mention in an interview, that in the 1st drafts of harry potter, hermione had a little sister.