Plausibly Deniable Snuggling
Tista enjoys his independence. Or at least his aura thereof. Therefore, he would never be caught snuggling or otherwise cozying up to one of his siblings...unless no one could see him because he was undercover. (Literally, of course)
Yep: That's Zora, visible and pleased. The lump just behind her? Tista, pleased at being so snuggly with his sister while being invisible, thereby maintaining plausible snuggle-deniability.
They're weird, but we loves 'em wholeheartedly!!