Everyone has always assumed that Mufasa and Scar were brothers, and that Scar had committed fratricide (which has scarred generations of children by the way). According to Rob Minkoff and Don Hahn, the director and producer of the Lion King, this is not the case. Not in the genealogical sense anyway.
The Lion King, inspired by nature
In an interview with the Hello Giggles website, the two men said:
In making the film, we talked a lot about the fact that Scar and Mufasa probably did not share the same parents.
The relationship between Scar and Mufasa is therefore directly inspired by the way lions behave in nature. The two men explained:
Occasionally, some groups have two males, and it's an interesting dynamic because they are not equal, one of them is in the background.
Scar would have acted like any lion in nature would have, by killing the dominant male in order to take his place, and then trying to suppress his offspring.