I recommend to check the history of crime. There is a lot of knife murders committed by people who were totally unprepared, not particularly strong and commited them on impulse.
Breastbone is an inch wide, more or less, so that leaves a lot of space on the chest to stab without trying to pierce the sternum. But even if the perp was attempting to stab them through the breastbone, it is less than half of inch thick and, because the victims were fairly young, their breastbones weren't fully ossified yet. Therefore stabbing them in their chests might have required far less strength than you imagine.
The victims were asleep when they were attacked and at least two of them were under an influence of the alcohol. You can easily kill someone with just one stab in the chest area - heart, thoracic aorta, abdominal aorta, take your pick. It's even easier to incapacitate someone with one stab - windpipe, lung, one of the smaller arteries and the victim is practically unable to fight, or to scream. Yes, according to the coroner the victims had defensive wounds, but if does not mean they were actively fighting. It was enoough to wave their arms incoherently to acquire some defensive wounds.
The reason I stated what I said in my post was because the coroner stated that the “all victims suffered upper chest stab wounds with one fatal blow.”
Also, it may be easier to incapacitate one sleeping person, but four would be difficult and rife with risk and uncertainties.
It takes a lot of strength and effort to kill in that way, especially doing it 4 different times.
I agree that a sleeping person is an easier victim than an awake person but the negative variables begin adding up after each victim.
This, IMO, takes skill in preparation and planning.
It’s also still very personal.
Of course there are *many* and most crimes involving a knife it’s by someone totally unprepared and usually used as a weapon of opportunity taken from the kitchen block, not brought to the scene, which again, says something about the murderer— It says, this was not one of those cases that the motive was robbery and the perp grabbed an object out of desperation.
Most murders involving a knife aren’t planned the way in which this was planned as evidenced by the fact there is no POI— which also goes to the level of organization and sophistication of this crime and the planning that was required to evade detection three weeks out, so far.
I agree that the defensive wounds may or may not have been an indication of active fighting, but more reflexive, but even so, the killer still probably had to maybe straddle them, or overpower them while silencing them in some way to accomplish multiple stab wounds in a very occupied house as to not alarm the others.
