Question: In the opening scene, why was Luis not particularly bothered by the sudden appearance of a neighbor child in their house standing at the entrance to their bedroom? He reacts to her initially as though she were an overnight guest who had been sleeping down the hall and not as someone who doesn't live there.
Answer: He also likely knew her since Anna had a conversation with the kid earlier about "say hi to your mom for me'.
Answer: Since it's not addressed in the script or the film, any answer would be speculation. One could be that Luis is just a generally laid back person, and the appearance of a young child he knows is not enough to ruffle his feathers, even if she isn't supposed to be there. He also just woke up, so he could be a bit groggy and not fully processing the situation as quickly as he might normally. More likely, it's simply the way the scene was filmed; the director and actor didn't think it important to show Luis "bothered" by the girl's presence (prior to noticing the blood, of course).