Ayooo that hugging scene felt ominous with that pose
I noticed that it was different from before, but I think it could just be that their roles have loosened up? Haruyo is the one who is more vulnerable in the interaction, the younger girl who wants reassurance, while Kasumi is the more mature girl indulging her. So Haruyo does not pull Kasumi's head to her own shoulder like before, and instead snuggles her face into Kasumi's shoulder. Her hands approach Kasumi 'from below' to wrap around her waist, instead of 'from above' around Kasumi's neck.
Haruyo acts less like the dependable older sister in general (other than the speech patterns) in this chapter, and Kasumi doesn't try to correct her. Haruyo is asking for hugs, not offering them. She's happy to accept help with studying. Kasumi is probably uncertain about their future but I think she's warming up to the concept of being someone Haruyo can lean on too.
As for why Kasumi's arms were hanging loosely while Haruyo was hugging her, instead of around Haruyo's back, or what it means, I can only guess. She's clearly not uncomfortable with the hug itself. Maybe it suggests her ambivalence, that she doesn't share Haruyo's optimism, or that being depended on is still a challenge for her.