Dear Emma,

I am sorry you are worried.

Usually what you have described is misdirected aggression. What happens is one cat sees, hears or smells something that frightens her. Since she cannot reach the offender, she takes it out on whoever is nearby.

Read more here:

http://www.aspcabehavior.org/articles/101/Aggression-in-Cats.aspx

http://suite101.com/article/types-of-cat-aggression-a72267

Since you are up high it could be as simple as a noise, a bird in a tree or a scent that humans cannot smell.

One other possibility is that she has a pain that she blames on her brother.

Look over both cats carefully. Feel all over their bodies and look in their mouths for any abnormalities.

You can always tell if a cat feels well by her appetite because sick cats stop eating first thing.

If you do not see anything but Maggie continues to get upset on the balcony keep them in until they both calm down and are getting along fine.

You can also get a feliway plug-in. It goes into an outlet like glade and Is calming to cats.

You can dab one drop of cooking vanilla on their tails. Once they smell the same they should go right back to getting along fine.

Feed them tasty food side by side like tuna so they feel good when they are together.

Please let me know how things go. If you have more questions just press reply and I will be here for you.

Warmest best wishes,

Terri