Is it ok to sit in the front passenger seat when the driver is your brother-in-law?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

In principle, a woman is not permitted to sit in a car with her brother-in-law if there is no third person with them. The evidence for this is the Hadeeth in which the Prophet said: “No man should stay with a lady in seclusion except in the presence of a Mahram (unmarriageable male relative).” [Al-Bukhari and Muslim]

The Prophet also said: “No man ever stays alone with a woman but Satan becomes their third (companion).” [At-Tirmithi]

In another Hadeeth the Prophet said: “Beware of entering upon women.”; they asked: ‘O Messenger of Allah! What do you say about an in-law? (men who are relatives of the husband like his brother, uncle…etc.)’ He replied: “He is death itself (i.e. his seclusion with women is as serious and dangerous as death).” [Al-Bukhari and Muslim]
If there is a third person with them in the car, it becomes lawful, but it is better for her not to sit beside the driver who is not her Mahram. Instead she should use the back seats.

Allah knows best.