"Are you her brother?"
Translation:Jesteś jej bratem?
When you ask a Yes/No question.
"Czy" is optional, though. It could be used here, but as you see, it was not.
Why is it jej? Shouldn't it be the instrumental case of the third person singular (feminine) pronoun ona (so it should be nią?) Since bratem is in the instrumental case, shouldn't the case of the pronoun also agree?
Or is it an exception since jej is a possessive pronoun (so it's always the genitive case)?
Possessive pronouns for 1st and 2nd person undergo declension, but those for 3rd person always have just one form. So "her" as in "her dog" or "her brother" will always be "jej", no matter of the case the whole noun phrase will be using.
"brat" is the basic, Nominative form.
"bratem" is Instrumental. It is mostly needed in sentences like "He is her brother" and also when something happens "with her brother".
You wouldn't happen to have a website that shows like...a grid of the tenses and when to use them would you?