Jackson already knows that Nathan is South's uncle, but he doesn't know the relationship between South and Nathan.
The only one who knows is Caleb.
When Caleb hears Jackson mention Nathan, he lowers his head, pondering something at the moment.
Rosalind agrees, and says to South:
"Right, invite your uncle over, after all, the military equipment used for this filming was provided by him. We should thank him from both obligations and courtesy."
South is still debating whether to invite him when Nathan's call comes in.
"Who is it?" Jackson asks.
"My uncle."
"Pick up, and ask Commander Foster directly," Jackson says.
South answers the call, and Nathan's voice comes through the phone.
"Hey, is everything done?"
"Yes, it's done." South hesitates for a moment before saying: "Tomorrow I'm hosting a dinner for everyone as a celebration. Uncle, they say to invite you over, are you free tomorrow?"
"Oh? Celebration dinner? Is everyone going?" Nathan asks.
