What is the ruling on benefiting from the financial return generated by an amount deposited by the sister in an Islamic bank, and does purchasing a car with it fall under the category of usury (riba)?

If the bank adheres to Sharia principles, then you have done well by depositing your money in it. The return belongs to the owner of the money, and you are not permitted to dispose of it without their permission. Disposing of it without permission is an infringement upon the right of another and an unlawful consumption of wealth. Allah has forbidden this, saying: "And do not consume your properties among yourselves through falsehood."