Staying true to your narrator's voice?

I have started my second book and I am telling the story from another character's point of view. My first character, Piper, is a sassy mouthed, do then think, character. My second character who is tell the second book is Ryder, level-headed and not mouthy. It's very different and I worry that Ryder's voice is going to sound very much like her sister's. And being sister's they should have a few similarities but I do not want Ryder to turn into Piper.

I'm also reading a fellow blogger's book, which is great btw! And she is telling the story from a girl's point of view and from a guy's point of view. She does a great job "being a boy." And I wonder how that was when writing. Like, I know how I would sound if I were to reply, but boys reply differently.

This lead me to think of all the different, countless voices that are seen in literature and how much work a writer has to do to stay true to the narrator's voice.

How do you stay true to your narrator?