Yep, that's the brat strap! I asked the artist to add it, it sounded super hot and fitting for her. The way the sweater is askew, revealing just enough of her shoulder and collarbone to imply things without revealing any boob, is IMO hotter than if she just had her tits hanging out and way more fitting to her.
This update has been VERY good for Sarah, lots of writing/relationship stuff and her CG!
I try not to pick favorites in terms of 'this is the girl the author clearly wants you to waifu,' if anything Celica is probably the closest there, but I do have a soft spot for this cute lil succubus as well.