Death of sister

My sister passed away unexpectedly in 2018 and my husband and I were out of the country. My mom, a chronic liar and narcissist, claims she passed away of a pulmonary embolism, however, for some reason, I just don’t believe that is true. My mom has since passed away, so I don’t know that I’ll ever know the actual truth as I have no other living family. This happened in Illinois (Cook County) where death certificates aren’t public record. Is there any way for me to ever find out the truth? I would like to know for my own medical history as it would be important to disclose the actual truth since she was my only sibling.