Here’s what TV Tropes says on the matter:

The Lion King may or may not contain a half-sibling example with Simba and Nala. While any shared parentage is never made explicit, the movie never addresses the fact that Mufasa, as king of the pride, would normally be the one to father any cubs, and that Mufasa and Scar (who is stated never to have fathered any cubs) are the only male lions anywhere to be seen. The spinoff cartoon “The Lion Guard” showed that Nala’s father is an unrelated male but it contains inconsistencies with the films and Disney doesn’t acknowledge spinoffs or most sequels, so it’s still rather ambiguous who Nala’s father is.

Surprised, everyone?