It's very sad.
Always bear in mind that someone could have a "pre-existing condition" and have no clue. The fact that this has happened twice in recent history on this ride is attracting a lot of attention, due specifically to the location. People die from pre-eixting conditions while driving their car, jogging, or shoveling snow. The fact that it has happened on a ride at WDW is a major media focus.
I rode MS with my DS and DD and we all loved it and felt no ill effects. Many, many people do feel ill after this ride (thus the barf bags). I'm not a doctor, nor do I play one on TV, but I can't draw a conclusion between motion sickness and death.
Meanwhile... prayers go out to this poor woman's family.