Liz, a couple of things about this fic.
1) I suggest you work on the dialogue. It seems rather helter-skelter and does
nothing to drive the plot forward. If you were going to use this much dialogue,
I would have suggested to write in a script format.
2) Something that occurred to me is that everyone seems to be too willing to
admit that Ryouga & Ranma are twins. I'd suggest that you get a skeptic in the
story, perhaps Akane or Genma. It would make the story seem more interesting.
3) On that end, I would also suggest you force Ryouga & Ranma to go through a
DNA test. Something to add to the plot's value.
4) On the good side, your spelling and grammar are excellent.
5) It would probably be a lot more readable if you break up your paragraphs a
bit. My suggestion would be to some of the dialogue as separate paragraphs,
especially the longer quotes and the back and forth dialogue. You seem to want
to have a fast pace with your fic, and this is one way of doing that.
6) Another suggestion if you want a faster pace fic is using action verbs
instead of passive verbs. Passive verbs drag down the pace of the fic.
7) Overall, I think the biggest problem with your fic is your treatment of the
plot. This is a very old plot idea (in fact, I've written a fic on the very idea
of twins or a long-lost-relative-who-is-right-under-your-nose fic). I strongly
suggest that you take an different angle of the fic. Put something into the fic
that you'd normally not see in such a plotline. I'm probably pretty guilty of
clicheing my story, so I'm probably not the person to suggest what kind of twist
you might want to put into a long-lost-relative story.