It doesn’t fit the pattern of M.B. P because there are 2 other healthy kids and you usually don’t see both parents with MBP, just the moms.
A father who is unaware of the mother’s MBP and who supports his wife could very well be an unwitting participant in this particular kind of abuse.
Also, I saw some mention of an older sister who supposedly has a mitochondrial disorder—so there is some evidence that another child has been affected.
I would also expect that as the mother’s psychiatric disorder progresses, its manifestation would become more severe and sophisticated. Thus, the older child might have received a diagnosis of mitochondrial disorder after many visits to many doctors, and the mother would have used that experience to refine her technique on the younger child.
Please note that I am only saying that Munchausen’s is a distinct possibility here, not that it is a definitive explanation of what is going on. We do not have the full facts here and have no clear basis on which to determine what is really going on.