Nabiki's behaviour makes no sense to me. Obviously she can't pass off
the engagement to Akane since Akane is totally irrational, but I can not
imagine why the prospect of having a bubblehead with wings and fangs as
a sister in law is so appealing to Nabiki that she would reverse her
original decision on Ranma's viability as a spouse. What's in it for
her? Why would Nabiki be impressed that Ranma is protective of his
sister?
Now Kasumi would make a certain amount of sense since she has the
opportunity to see Ranma's all round admirable character, and might feel
for the poor girl who is obviously as nice as nice can be.