We did want the siblings to have a disagreement anyway, maybe Meela was the behind the scenes turning point that make Xana and Xano drift apart from one another. I know we wanted the girl to have fire bending and him to have water and be the more assertive one but I keep forgetting which name is which. I though Xano was the boy and Xana was the girl right?

I assume one needle would put the disease into the bender and as their system is weakened, they sneak another injection which extracts the bending itself, as well as the life force. So in that regard, we could have Xana not go back for Meelo because she is doesn't agree but maybe she extracted Pema first and some air bending was picked up and thus she didn't think they need to use Meelo too, thus he recovers on his own without the second secret visit?

Would that make sense?