A: By brother-in-law we assume you mean the husband of your deceased sister. When your sister said that her husband could have the car after her death, it meant that she had made a Wasiyyat (bequeathal) in his favour. And according to Shari'ah one cannot make a Wasiyyat for an heir (Al Jauharatun Nayyirah V6 P182); husband and wife inherit from each so they are not permitted to make
If she had given him the car during her lifetime, he would have become the lawful owner, in which case the car would not form part of her estate after her death. Because she had not done that in her lifetime, now the car will be regarded as part of the estate. All heirs will have shares in the vehicle.
Allah Ta'aala knows best.
Checked and approved by: Mufti Siraj Desai