The Voices in My Characters' Heads

In my work in progress (WIP), my story is told from alternating first point of view perspectives. Twin brother and sister take turns telling the story. I have the first 2/3 of the story, but am slowly realizing that I lack character voice. (Yes, I'll admit, sometimes Siren is right). My voice is shining through, but the chapter told by one character should sound different than that of the other simply because we all have different voices and perspectives and personalities. So my current project is to research voice and add it to my WIP. Here are some articles I found for anyone who may be interested...

Layering Powerful Voice to Create Memorable Characters
Character Voice Writing Exercises
How to Use Your Character's Voice
Giving Your Character Unique Voice

Basically, what it comes down to is you should be able to tell who is talking based on their accent, vocabularies, etc, without having to look at the speech tags and without being cliched. Easier said then done. To me, character voice is like the icing on the cake--the story is the substance but the voice makes the reading so much more enjoyable.

Anyone have any insight on creating character voice?

Back to writing I go...