I don't understand how you're supposed to solve this. I wrote out all the answers (which I'm about to cut and paste from someone else's post on this forum) but I don't see how you come to (SPOILER) Ralph/Red as the answer.

The raw data is:

Bart R: My sister tells the truth when she is blue.
Bart Y: Some blue sprites always lie. Some always tell the truth.
Bart B: Yancy and I are like the most other sprites.

Betty R: The sprite who can make it safe for you to cross can only do so when the first letter of its name is different from the first letter of its color.
Betty Y: The sorcerer Sanwe is evil.
Betty B: Only a blue sprite can make it safe for you to cross.

Jane R: When I am blue, the sprite who can make it safe for you to cross is red.
Jane Y: My brother tells the truth when he is blue.
Jane B: Some blue sprites always lie. Some always tell the truth.

Jim R: Yellow sprites always lie.
Jim Y: My sister tells the truth when she is blue.
Jim B: The sprite who can make it safe for you to cross can only do so when the first letter of its name is different from the first letter of its color.

Rachel R: My brother is either Bart or Yancy.
Rachel Y: When I am yellow, the sprite who can make it safe for you to cross is red.
Rachel B: Red sprites always lie.

Ralph R: Only a blue sprite can make it safe for you to cross.
Ralph Y: The sprite who can make it safe for you to cross can only do so when the first letter of its name is the same as the first letter of its color.
Ralph B: Unlike most other sprites, my sister's name doesn't start with the same letter as mine.

Yancy R: My sister lies when she is blue.
Yancy Y: There are eight of us, as you can see (the Guard's don't count). Four of us are boys, like me. Four are girls. We are four pairs of brother and sister.
Yancy B: Yellow sprits always tell the truth.

Yvonne R: Only a yellow sprite can it make safe for you to cross.
Yvonne Y: Only a red sprite can it make safe for you to cross.
Yvonne B: Only a red sprite can it make safe for you to cross.

At first I thought all the Yellow sprites tell the truth, but that's not so. In which case, how are you supposed to solve this?

It seems that you need to determine which statements are True (T) or False (F).

But I can only get this far:

* Betty/Yellow ("The sorcere Sanwe is evil) is true
* Yancy/Yellow ("There are 8 of us..etc) is true
* therefore Jim/Red (Yellow sprites always lie) is false.

After that you have to start making some assumptions. As I said above, I assumed that yellow sprites always told the truth. But that wasn't correct.

You solved it the same way I did - guess work.

I'm trying to do this the proper way too. I've only found two other things I believe to be clear lies.

Yvonne - Red says it's safe if you speak to her. She also says "Only a yellow sprite can it make safe for you to cross."
That's a contradiction, as if her second statement was true her first would have to be a lie.

Jane - Red also says it's safe. And she says "When I am blue, the sprite who can make it safe for you to cross is red."
Another contradiction. If the sprite that can let you pass is red, and she's that one, she couldn't be blue at the same time.

Don't bother, not even try to solve this puzzle logically, all will be in vain. At first I thought there's a way to discern pairs of brothers-sisters by eye (what if they change color simultaneously?) Then, after hours of horrid repetitive shell game, found out: with every screen revisiting any specific pair of sprites alters their conjugation chaotically. In particular, it means that statements of:
Jane (red) "When I am blue, the sprite who can make it safe for you to cross is red"
and Rachel (yellow) "When I am yellow, the sprite who can make it safe for you to cross is red"
are Randomly true/false.
This nonsense is aggravated by vagueness of starting assumptions.
Bart (blue) "Yancy and I are like the most other sprites" - is it a hint that they don't grant safe passage?
Yancy (yellow) "There are eight of us, as you can see. Four of us are boys, like me. Four are girls. We are four pairs of brother and sister" - but we've been confused even about sex of single faerie at previous screen! Is male sprite allowed to have another brother or have to stick to sister?
Overall, game creators messed here. I would rather see that one hermaphrodite faerie speaking some rules (smth like "one colour always lies, second is frank, third's dubious"). But you have to die, die trying all options

mikhailbeaver, don't lie to the kids. It's perfectly possible to solve it by yourself using the clues. Everyone else had to do it. I had to do it. There are three colors. The third color is implied to be up in the air as to whether it's true or false. You have to find out what's definitely true and defnitely false. That's all.