Sina is Dalish, and her clan is small; bathing is done when and where it's convenient, by whomever is at liberty to do so. She has little shyness over disrobing, for the most part only dressing so modestly because she gets cold easily.
Following Thranduil's request, she steps gingerly into the water, removing her nightshirt as she does so, and lowers herself in with a weary sigh. Now that she's submerged in the hot water, she can't not appreciate its comfort, and it lifts her spirits slightly. The shard glitters oddly over the surface, refracting in patterns of light all around the tent. Sina watches the glimmering water as though mesmerized, in a despondent sort of way. It's beautiful, this thing that's killing her, this thing that has killed others like her.