You have come such a long way in getting the visa, isn't it a shame to
let something like that get the better of you? Don't lose something
that you have waited so long for, just for the lack of trying. Staying
with Mum and keeping the visa is still possible.
Perhaps proper
documents from your mum's doctor to confirm her medical condition might
do the trick. No harm giving it another try. Your case officer has
rules and policies to follow and she does not represent the whole of
Oz.
Get some proper doctor's letter and try again. Hope Mum gets
better soon.