So sorry about your sister-in-law.
Warfarin requires routing monitoring of something called the INR (international normalized ratio) which is a confusing way to refer to clotting time. Suffice it to say, warfarin is very diet sensitive with vitamin K being the main variable and it is not difficult to get one's blood "too thin", or "too thick" depending on one's diet and adherence to the monitoring regimen. I'm not saying that this was necessarily the case but it's always a possibility with warfarin. It is borne out in the statistics of Eliquis being superior in that the bleed risk is lower.
Also, hitting your head is bad no matter what. Plenty of unfortunate souls have met their end that way absent any blood thinners. Be careful!