Sorry for your SIL's distress. My understanding is that stroke is less common than heart attack in cats. Perhaps this will help:
Q: "I had a 5 years old male cat that just died in the middle of the night, he was always healthy. ..."
A: "... The most common reason for sudden death in a younger otherwise healthy male cat is a heart problem. It is the type of heart disease where you generally don't hear a heart murmur. The walls of the heart become thicker making it more prone for blood clots to form on the valves of the heart. Sometimes a clot comes loose and can travel to the lungs causing sudden death.
Here is a link for you to read
http://www.sniksnak.com/cathealth/hyp-cardio.html
There is nothing that you could have done. I hope this helps you understand some."
" ... Incidence can vary in different parts of the country, probably because of the gene pool present in a particular area. The most common problem among cats is heart muscle disease (cardiomyopathy) and it causes no end of trouble. ...
... Research indicates hypertrophic cardiomyopathy may have a genetic component and can affect cats at any age. However, hypertrophic cardiomyopathy is most common in young cats more than one year old and in middle-aged cats. It bears repeating, though, this disease can strike any cat at any age.
In hypertrophic cardiomyopathy, the muscle wall of the heart thickens and stiffens, so the heart's chambers are greatly reduced in volume and can't relax properly after contracting ...
The severity of symptoms varies from cats who appear totally unaffected
to those who die suddenly. A mildly affected cat could very possibly lead a totally normal life. These cats may be identified only when symptoms suddenly develop due to a stressful event, such as bathing or teeth cleaning."
Hopefully, no one was trying to teach your cat to roll over and play dead ... that only works with dogs. Perhaps other stressors (e.g. being at your SIL's place and around other cats) triggered an emotional event in your cat which was already predisposed by a heart condition to pulmonary embolism.
I'm not a Vet; however, I have performed necropsies. Based upon the history you've given, it doesn't seem to me that a necropsy is indicated for the health and welfare of other animals your cat may have contacted. The Vet your cat was taken to should have been able to advise you in this regard.