This was dub only, as 4Kids wanted to do away with the notion that both of Takeshi's parents seem to go off whenever they please, leaving all of their kids by themselves. At least with a supposed death, it gave Flint a reason to be in a sort of grieving process and step away from him home life.
But none-the-less, it's a dub line only. She never died in the original. At the time, 4Kids never anticipated that the Japanese writers would eventually bring her into a story.