ALL MY OPINIONS ONLY
I think Xana was downstairs accepting her DD order right around 4:00 am, then came up the stairs to the 2nd floor and her bedroom.
I think she probably ate in her room only because BK snuck in that slider door in the minutes after she got her food.
Do we know that Ethan didn't eat too? Do we know Ethan was sleeping while Xana ate?
I think Xana probably finished her food and had put the bag in the kitchen and was back in her room on her bed scrolling when she heard something upstairs.
Could she have woken Ethan and said "I think someone's here"?
I am not sure what time that was said. Maybe Ethan got up and stuck his neck out of the room trying to ascertain what was happening.
DM said she opened her door more than once to peek out. Maybe Ethan did the same but BK saw him/them as he was coming back downstairs to the 2nd floor and came towards Xana's room saying he was there "to help them"
Maybe this is what BK was focused on so he didn't even notice DM.
We don't know if either Ethan or Xana said something to BK before he harmed them. They could have been startled by this unknown person's presence and said something quietly, like "What are you doing here?" "Who are you?" "Why are you here, it's 4:00 am" or "Who are you visiting?"
Alternately-
Xana could have seen or heard BK coming in through the slider into the kitchen and heading upstairs when she was coming upstairs from getting her DD order and said "I think there's someone here" loudly enough for both DM & Ethan to hear alerting Ethan.
Maybe they decided it wasn't a big deal after seeing him and Xana finished eating her food but they were still keeping an eye/ear out when he came back down after murdering MM & KG.
I don't think Xana was sitting in the LR or in the kitchen eating, only because BK entered and exited through the 2nd floor's communal living spaces twice in less than 20 minutes. In that case I think the battle would have been in the LR or kitchen rather than Xana's bedroom.
I think BK came to XK & Ethan's room just like he went to where MM/KG we're sleeping.
I am also of the mind that BK had been stalking the house and MM & KG specifically, and I think when he was in the neighborhood, he would drive to the elevated lot behind the house so he could see into MM & KG's rooms (when KG lived there) AND that BK was aware, from their social media, of their movements and activities, including that night they were seen on the public live video at the food truck before going home.
That's my take at this point in time based on what is publicly reported information and what I have gleaned here on WS.
JMO