Oh, no, please tell them *exactly* what they're getting into. Any deceit on your part should get you kicked out of the sisterhood. Hmm...in the novels you would also wear the same type of jewelry (ring, bracelet, etc.) with some symbols on it recapitulating your motto.
As you can see, Mme. Drake does translate
iuncta iuvant as "in union there is strength." I wanted you to be aware that the translation was loose. Please suggest this one to your sisterhood as one option, and cite the source (Mme. Drake, not necessarily me).
EDIT: If this is a sisterhood, you could also do something like
iunctae iuvant (specifically feminine plural), or
iunctae iuvamus, "together we help."
http://books.google.com/books?id=919DAAAAYAAJ gives the moral of "The Farmer and His Sons" as
Haec fabula docet parvas res crescere concordia, magnas dilabi discordia...."This story teaches that small things grow by concord; great things fall away by discord." Mr. Clarke doesn't claim it means "in union there is strength," so that's further away from what you want.
You may want to echo the form of
in vino veritas, "in wine (there is) truth." So you might want to drop the
est "is" from Johnny's phrase.
If you wanted a word that looked more like
veritas, you could use
potestas. I was hoping to find a shorter word for "union." You could use
unitas, in which case you'd have:
In unitate potestas.
unio is Late Latin -- many around here prefer the classical era though. If you like this word you could do:
In unione potestas.