From Saba's perspective, Lugh is the greatest thing since sliced bread. He's the GOAT of twin brothers. He's The Force Awakens of people.
In other words, sis loves him a whole bunch.
We mean, really, the entire novel is about Saba's quest to rescue Lugh, which is a neat little subversion of the classic "damsel in distress" motif: this time, the damsel is saving the dude. But if we look closely at this tight sibling bond, we'll come to see that everything isn't quite as rosy as it seems at first glance. Check out this nugget from the novel's opening passage, for example:
Lugh got born first [...]
Then me. Two hours later.
That pretty much says it all
Lugh goes first, always first, an I follow on behind.
An that's fine.
That's how it's meant to be. (p.1-7)
As you can see, Saba acts like Lugh is inherently better than she is, which is so wrong. We're sure that the kid is great and all, but Saba is 1) a powerful warrior, 2) a rugged survivalist, and 3) a surprisingly good leader. She has tons of amazing qualities. When we look at it this way, we find ourselves agreeing with Jack that Saba's praise of Lugh "sounds too good to be true" (8.53). She's measuring herself up against a standard so high it doesn't actually exist.
On top of that, we could point out a few less-than-admirable traits Lugh has. There are his statements that "love makes you weak" and "carin fer somebody [...] means you cain't think straight," for example—which is a pretty messed-up thing to say to your twin sister, who adores you (1.101). We see another splash of Lugh's selfishness when he blows up on his dad about the foolishness of the old guy's mystical beliefs. He might have a good point on that one, but he handles it very poorly—and it's possible he's even downright wrong.
Either way, we never get to know the real Lugh: at least in this installment, we really only get Saba's perception of him. (Seriously, the dude spends most of the novel out of sight and incapacitated.) Although the twins are eventually reunited, it remains to be seen whether their bond will stay strong now that Saba has grown and changed so much.
There's one good sign that comes at the end of the novel, though. The scene: our newly reunited siblings are heading off on their next adventure, with Lugh in the lead, just like it's always been. And lights, camera—action:
Hey, he says, what're you doin back there? I ain't got a clue where we're goin. Git on up here an lead the way. (9.1123)
As you can see, this is a reversal of the novel's opening. Just scroll up if you don't remember it. It says a lot about Lugh that he doesn't get jealous of Saba, but instead recognizes his sister's growth and admires her for it. He might not be as amazing as she says he is, but he's still a pretty swell guy.