Cookie has a unique sort of relationship with Fraise's friends. They tend to think she's weird and like to bug her when they can, because that's just what people do to big sisters. It's a love-hate sort of thing; Cookie's plenty happy that her sister gets along with so many people though. Honestly, even if Cookie doesn't have a lot of friends she'd want Fraise to have all the friends she could ever want.
But there is a love part to the love-hate. She can get along with those rascals if they behave just so. The story here, as the artist states, is that Schmucky has decided to spend the night in the Souris house as part of a sleep-over Fraise is having. It's plenty fun, but eventually it's time for bed and they all get snug as bugs in rugs.
Sleeping too close to Fraise tends to make people hot though, and sweaty. She's kind of smothering! So poor Schmucky wakes up half buried and feeling too hot for comfort. He wiggles free and heads to the bathroom. A thunderstorm rages outside the safety of the household, and he doesn't notice right away that the power has gone out. He takes a little light with him and toddles on off to the bathroom to get out of his clothes.
He didn't know Cookie was in there, but he will. She's gonna scare the bejeezus outta him.
OooOoOOOoo, I'm a scary witch that eats childrennnn, wOOoOoOOooooOo.