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.