Nuka was born before the Outsiders came to Pride Rock Scar saw that Nuka was both physically and mentally unstable.

Luckily, Zira wouldn't give up hope. One was a girl. Which Scar didn't approve of. She then went out again she stole Kovu who was just born. They attacked Zira. But she killed both the parents.

"Born in Grief, Raised in Hate, Helpless to Defy his fate."
-line in "one of us"

Zira didn't tell Kovu he was adopted until he was older. All he told him was Scar wasn't his father. Until one day he asked who his real father was and Zira had to tell the truth that she wasn't the real mother.

All lions (including the outsiders) weren't allowed to leave Pride Rock but Scar made an exception for Zira to find a good heir.