What is the ruling on a girl traveling for approximately a day and a night without a mahram, but in the company of her cousin and her cousin's husband in a private car?

It is not permissible for a woman to travel with her paternal aunt's daughter's husband without a mahram, even if she is accompanied by her paternal aunt's daughter, due to the general nature of the Prophet's (peace and blessings be upon him) saying: "A woman must not travel except with a mahram."