Monster SIL is back

OK, Here is my feeling.
Typically front seat is more convenient than back seats.
If you are going for short distance is ok. But long drive the back seat is more boring places to sit.
Here are two things don't think you SIL only interested to sit next his brother may be she likes the front seat in the car. Since it's her brother and her family (most of the times unmarried women in the house gets lot of pamper), she might now even thinking that she is stealing your seat.
If it is once in a while think why do even worry. Let brother and sister enjoy and anyhow you will have rest of the days with you husband. Just my 2Cents.