I'm torn. I dislike cutesy twin-themed names like this. But I'm addicted to making suggestions. Oh well . . .
Jacinta /
Jacintha /
JacindaJulie /
Julia /
JulietJenniferJocastaJuno /
JuneJosephineJohanna /
Jane /
Jean (
Jeanie) /
Joan (Joanie /
Joni) /
JeanetteJohari
JoyJamilaJudithJemimaJada /
JadeJustine /
JustinaJuanita (this would at least break up the cutesy same sound aspect)
Bottom line: Anything but
Jacqueline /
Jacobina /
Jamesina / Jamesetta. Those are the feminine forms of
James (=
Jacob /
Jacques) which would be creepy for siblings.