Just because Doreen is referred to as Sister by Masa and Mune does not imply that she is also a dream of Melchior. They are probably saying it because she is another dream-being like them and therefore their sister in that sense.
It does imply, but it doesn't necessitate. Does anyone know how strict Japanese is with this sort of thing? That is, would a Japanese individual use the phrase "nee-chan" in a general use, such as to imply members of a similar group (sort of like the English phrase "sorority sister") or can "nee-chan" only mean actual sister?
However, looking over the script I can't find any definite connection that Melchior alone created the pendant. The game actually says that the Gurus (plural) made it.
Here are a few seemingly relevant quotes I found through the translation page, for discussion's sake:
[NA Doreen, Enhasa]
This is the eternal kingdom of Zeal, where dreams can come true.
But at what price?
Am I a butterfly dreaming I'm a man... or a bowling ball dreaming I'm a plate of sashimi?
Never assume that what you see and feel is real!
I'm Doreen.
Seek the hidden path, and open the doors of knowledge, each in turn.
[RT Doreen, Enharsa]
This is the eternal Magic Kingdom Zeal. The place where all desires come true......
However, there's no telling how high the price for that will reach......
The world that you see with your eyes and the world that I see with my eyes may be completely different things.
Listening? All that exists in the universe are the destinies of lives. Don't think that only what you can see and touch are reality.
I'm Doreen. Seek the way that was shut. Get the order correct and open the door of knowledge.
***
[NA Mune, Zeal Palace]
Big sister Doreen's at Enhasa again
[NA Masa]
Yeah… That's 'cause she likes dreams.
[NA Mune]
I like the wind better! Whoosh!
[NA Masa]
I wonder how ol' man Melchior is... Humans are so odd...
[RT Leon, Zeal Palace]
Doreen nee-chan went to play in Enharsa again.
[RT Gran]
Yeah...... It's cause nee-chan likes dreams.
[RT Leon]
I like the wind better. whooshwhoooosh!
[RT Gran]
Wonder what he's doing, old man Philosopher of Life
Hasn't been around lately...... Sheesh, humans......
***
[NA Young man, Zeal Palace]
Schala’s pendant was made from the same red rock as the Mammon Machine.
When she chants, the pendant glows.
[RT Young Man]
They say Sara’s pendant was made from the same red stone as the Demonic Vessel.
I’ve heard that when Sara takes the pendant in hand and prays to the Demonic Vessel, it gives off a mysterious radiance.
***
[NA Old Man/Young Man, Zeal Palace]
That glow…?!
That’s the pendant the Gurus made for Schala!!
How did you get it?
[RT Old Man/Young Man]
Oh, that radiance…!?
That is without a doubt what the Philosophers made for Sara!!
How did you ….!?
She certainly seems more... aware in the Retranslation than the NA version. Curiously, Doreen's name doesn't seem to have been researched much yet. It is possible that there is a connection to Doreen Valiente, an early Wiccan and somewhat prolific writer of witchcraft in general (and a practicioner of magic). Or perhaps Doreen Massey, a geographer with some postmodern philosophies that don't seem too alien to Chrono Trigger Doreen's.
The name "Doreen" itself seems to mean something along the lines of "Gift" (from the Greek doron and the -een suffix). It is possible that this may actually be a play on words and that her name actually may mean "Gift of Eve" (-een being a form of evening, which can also be represented as eve). Eve being not only evening before, but also the biblical character. In genetics, "Eve" might be used to refer to a maternal ancestor and it is implied that Marle is related to the Zeal royal family (thus, Marle's pendant is the "gift" of her "maternal ancestor").
It could also be "Gift of Living One/Source of Life" as Eve itself means such in the original Hebrew (derived from Eva). As the dream species have a connection to dreamstone, and in turn the Entity, Doreen might literally be a gift from the planet.
Or it could be Gift of the Evening, as Zeal was certainly in its twilight hours.
Or Doreen might just mean jack squat
