lion king: simba & nala fall in love at the end but from the looks of it mufasa and his brother were the only males so simba and nala are brothers and sisters or cousins Amirtie?

I think they're siblings and
Nala's mother and Simba's mother are sister mates

I've looked this up once, the most logical theory was it was Scar since that makes sense for him not wanting to kill Nala as she was his daugther. Else when taking control of a pride they kill off possible threats and anyone that isn't related to them directly.

Another theory was that some lion from another pride impregnated her.