I don't think the author was clear enough, and he needs to put more emphasis, and be more explicit about it, but if you read between the subtle lines...
It wouldn't be wild to assume that she has no underwear at all
She just woke him up by loudly making lewd noises in her sleep while sprawling all over the floor like a kid. It's just a gag where she makes a ridicoulous spectacle of herself (again) and proves him right in not respecting her as an elder sibling.