I think so, IF they were still downstairs at that time?
It makes more sense that Kathryn and Nathan stayed in their bed in the basement after Jennifer left with the baby. If blood letting means an attack at the side door, then it makes more sense that Kathryn heard something and went to the side door.
It was pure speculation that they went to the upstairs bedroom at 11PM because there was really no other explanation for blood in the spare bedroom. However, an attack at the side door, and blood in the spare bedroom would suggest that Kathryn was the first victim at the side door, Nathan was the second victim in the upstairs bedroom (where his mother would have been), and Alvin the third victim.
I zoomed in on the photo of the blood by the side door and it actually looks like there are bullets on the floor - did anyone have a closer look at that photo?