Of course, you will still be liable despite the fact that it is not anymore in your possession because actually, when the motorcycle was delivered to you by the motor company, it is already yours despite not having paid in full. Remember what you signed when you applied for it? Usually, it is a deed of sale and a chattel mortgage. The absolute deed of sale gave you full ownership over the motorcycle. The chattel mortgage on the other hand, caused you to have loaned money from the motor comp and to ensure to pay for the loan, your motorcycle was used as security instead. Thus, thus the absence of the motorcycle does not extinguish your obligation to the motor comp.
Meanwhile, with regard to your brother, I can tell it's carnapping, but it is better that you go to prosecutor's office to determine really what case you can file because the facts are not complete, and we don't you to have limited options. .