Possibly. It's very likely that Vitani and Kovu at least have the same father, seeing as they seem to be the same age and are likely from the same litter (Kovu being the younger of the two, obviously).

Nuka... may or may not, I guess. I mean, I don't see any reason why he wouldn't, but at the same time if any of them are going to be half siblings then surely he'd be the one with the different father due to the age gap.

They pretty much have the same father who's a rouge or maybe another king who left Zira and her cub, Nuka to go find another pride. Zira who was pregnant with Vitani and Kovu moved on with her pride (the outsiders) and sees a new king of pride rock who is Scar. So, Zira and her pride lived with him. when the cubs were born, Scar notices Kovu and decides to have him as his heir. That's when Nuka became jealous of Kovu.

I personality think Nuka and Vitani were Scar's children however Scar's thought they would fail as heirs (Nuka being too weak and Vitani being a girl.) So Zira mated with Chumvi to make sure she had a strong heir due to Chumvi being stronger then Scar. Also Chumvi looked quite like Scar, so Scar would think Kovu was his own child.

OR Scar simply found Kovu and adopted him. That would also explain why Nuka is so jealous not only did his own Father replace him but with someone who wasn't even his own offspring.

^^ Chumvi is not an official Disney character. Zira very much had all her cubs by one male which is unknown.
Scar doesn't have any children in the movie sense. Nuka rather calls him Scar rather than dad or father that is heard from the film which shows Scar is not Nuka's father. Scar and Zira were never mates Zira is rather his big fan and very obsessed with him. That's why Simba exiled Zira and co by his father's laws.

I think Nuka is Scar and Zira's son, Vitani is Scar and Nala's daughter and Kovu is Chumvi and Zira's son.

nuka is scars son

Yes they do. Their father isn't Scar, it's an unknown lion.