If a woman is a Christian married to a Muslim man, should his sister and/or mother be required to at least make arrangements before coming to visit? Can they just get on a plane and show up at the home without notice? Also, is there a requirement or rule or something that states that a sister sits in the front seat of the car owned by the Christian wife? The sister is younger than the wife. What about the mother? Is there any respect that should be shown to the wife?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Islam places emphasis on many etiquettes that increase love, affection, and harmony among the members of a community. So, asking permission, respecting the elders, and honouring a person in his area of authority are some of the etiquettes legislated in our religion.

So, it is among the known and accepted ethics that your sister-and mother in-law should inform you of their arrival and should not make a surprise visit to your home. It is also the appropriate behaviour, and it is better for them lest they go through all the trouble of traveling and discover that you have a sudden journey, for example.

It is among the etiquettes of Islam that a person should be respected in his area of authority. The Prophet said: "When in the house of others, one should not sit in the seat of the host without his permission." [Muslim]

Though, the above narration states 'the place in the house' specifically, but the same ruling applies to a person's property such as a car, etc. Therefore, your husband's sister is recommended to ask your permission to sit in your place in your car for two reasons; it is your right, and you are her elder.

It is also among the noble characteristics that one should respect his guests and deal kindly with one's in-laws. So, if you permit your mother in-law to take the front seat, it is better for the two reasons we have mentioned above. On the other hand, you are not blamed if you do not permit her, since you are the owner of the car, and you have the Islamic right to sit in the front seat or in any seat you wish. But, doing so especially with the mother in-law is contrary to the rights that our guests have upon us in general, like respecting and honouring them and our in-laws in particular.

Allaah knows best.