taking care of Aunt with MS

This is a huge, and admirable, commitment.
MS is a highly individualised disease - many persons with MS don't need lot of care, or only during acute flares of the diseas; others are significantly disabled by it.
The Australian system is quite different to that in the US, so I can't give you any specific advice, just some food for thought.
The first thing I would take in to acount is the period of time that study (including clinical placements) would take you away from being able to provide care. The more advanced the course the greater the time commitment.
The other thing is that, for the most part, I wouldn't have thought that highly skilled nursing care would be required for day-to-day assistance. The families of many of our patients with long-term care needs (like advanced MS, significant stroke etc) have a sort of 'on the job' training - they rpovide care over a weekend, within the hospital, with nursing support, supervision and advice.
The kinds of things needed include pressure care, medication administration, assisted feeding (either orally or through a feeding tube), basic hygeine, and toileting.
A qualification in nursing would teach you about anatomy, physiology, neurophysiology, nutrition, pathophysicology, pharmacology, and the psychology of loss and illness (among other things), and also teach you clinical skills.
While these would certainly enhance your understanding, and perhaps improve the quality of your care, you may find that you invest a lot of time, effort and money without a marked increase in your abilities.
I hope this has been helpful, rather than confusing