Is zakat obligatory on money shared between me and my siblings, which I currently possess?

A person is not required to pay zakat on another's wealth unless the owner has authorized them, or if the other person is under their guardianship (due to minority or insanity). If your share of the joint wealth reaches the nisab and a hawl has passed, then pay zakat on it if you have other, non-joint wealth from which to disburse the zakat.