Hotaru wasn't entirely sure what to make of the fact that they spoke as if they had already adopted her, but she wasn't going to question it. Mostly, because she was afraid that if she did then they would leave and the more she thought about it, the more she realized that if this couple adopted her then she would be able to see Steven again like she had when he lived with their foster parents.
So, she did as Michiru told her and measured out the water, not saying anything as she simply listened to the instructions from the other woman.