Is it permissible for a husband to register his car in his wife’s name to provide money for her and their daughters after his death, knowing that he has sought permission from his sisters?

The question does not clarify what is meant by registering the car in the wife's name. If it is merely a bequest for her after his death, then this is a bequest to an heir, which is not enforceable unless the other heirs agree to it. The sisters' waiver of it during the husband's lifetime is of no value. However, if it was an immediate gift that fulfilled its conditions, then it is valid, and the wife owns it, even if others do not approve. Similarly, the sisters' approval of the bequest after the husband's death is permissible if they are adult, of sound judgment, and consent willingly.