Not long after Sans goes to sleep, Sokka makes his way to the living room where Papyrus is resting. He takes a seat on the floor, back to the coffee table, and watches Papyrus for a long moment. He's already decided what he's going to do, but that doesn't mean it feels great; actually, Sokka feels a little bad about. He shouldn't. This really is no different than defying a spirit librarian for vital knowledge or snooping in a dangerous woman's house to find out what she's hiding. This is about figuring out the threats they face and making plans to do something about them — and it's about protecting others. Sokka hasn't been here very long, but there are people here he wants to help, and regardless of his feelings about Sans' supposed status in this household, he has started factoring him into his plans, too. He can't do that if he doesn't fully understand what's going on.
But Sans didn't need to be explicit in his displeasure when Sokka started asking his questions. He didn't need to say anything at all for Sokka to see in him the same way he'd feel, if it were Katara lying on the couch. He tries to be a practical person, but even he loses his practical approach when Katara is in danger — like when he tackled their friend because he accidentally hurt her. He gets it, and that's what makes him feel guilty about this, like it isn't the same as outsmarting a spirit to end a war.
But in a roundabout way, this is also about protecting Katara. If Sokka can fill in all the blanks about why and how he was brought here — and if he can figure out who's responsible and do something about it — then he can ensure that Katara and their friends never come here. He has to ask, and it has to be sooner rather than later. This is an active threat, and he's wasted so much time already.
That doesn't mean he has to start out by being blunt. He would normally — it's what he did earlier, when he tried to ask a few questions — but he thinks about Katara saying, Sokka, with a chide in her tone and it makes him temper his approach. Instead, he offers a piece of information about himself that he's told no one here — not Sans, not even Bucky — in fear of drawing this place's attention to her. He says, quietly, "I have a sister," like it's an apology for the barrage of questions that are to follow.
Wildcard - lmk if I should make any changes!
But Sans didn't need to be explicit in his displeasure when Sokka started asking his questions. He didn't need to say anything at all for Sokka to see in him the same way he'd feel, if it were Katara lying on the couch. He tries to be a practical person, but even he loses his practical approach when Katara is in danger — like when he tackled their friend because he accidentally hurt her. He gets it, and that's what makes him feel guilty about this, like it isn't the same as outsmarting a spirit to end a war.
But in a roundabout way, this is also about protecting Katara. If Sokka can fill in all the blanks about why and how he was brought here — and if he can figure out who's responsible and do something about it — then he can ensure that Katara and their friends never come here. He has to ask, and it has to be sooner rather than later. This is an active threat, and he's wasted so much time already.
That doesn't mean he has to start out by being blunt. He would normally — it's what he did earlier, when he tried to ask a few questions — but he thinks about Katara saying, Sokka, with a chide in her tone and it makes him temper his approach. Instead, he offers a piece of information about himself that he's told no one here — not Sans, not even Bucky — in fear of drawing this place's attention to her. He says, quietly, "I have a sister," like it's an apology for the barrage of questions that are to follow.