From the Final Fantasy Tactics Battle Mechanics Guide:

1C [Delita's Sis]
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Younger sister of Delita Hyral. Supported each other since their parents died
at a young age.
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COMMAND SET: 23 MAGIC | SPRITE: Teta
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EQ: <nothing>
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IMMUNE: Crystal, Undead, Charging, Jumping, Defending, Performing, Petrify,
Invite, Darkness, Confusion, Silence, Blood Suck, Treasure, Oil,
Float, Reraise, Transparent, Berserk, Chicken, Frog, Critical, Poison,
Regen, Protect, Shell, Haste, Slow, Stop, Faith, Innocent, Charm,
Sleep, Don't Move, Don't Act, Reflect, Death Sentence
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INNATE: Auto-Dead
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HPM: 0 | MPM: 0 | SpM: 0 | PAM: 0 | MAM: 0 | Move: 0 | C.Ev: 0%
HPC: 0 | MPC: 0 | SpC: 0 | PAC: 0 | MAC: 0 | Jump: 0 | TYPE: Special
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She has no statistical growth and will always automatically be knocked out in battle; because of this, she is immune to a lot of status effects, enhancing effects, and cannot become crystalized.

Her command set is Magic.

23 [MAGIC]
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Delita's Sis
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(A) Cure - 50 JP
(A) Wish - 0 JP

JP to Master: 50 JP

She lacks the standard reaction, support, and movement abilities that every class in the game has. If you want to interpret it as a storyline point, you could say that Teta was not as far along in her classes as Alma and Ovelia, or that, at that point in the story, those girls had not learned the magic that we see them with later on.

Alma, Teta, and Ovelia are all trained in clerical duties. Through their studies of the scriptures of God, they are able to use a small amount of holy magic formed from pure spirit.

Magic Barrier is the manifestation of the will to protect a loved one. It bestows the target with a holy light granting numerous positive status effects.

Dispel II is the manifestation of the will to save a loved one. It dispels any impure status effects upon the target.

An interesting thing to note is that, while Teta never learned either of these abilities, she did have Wish, an ability shared only between Ramza, Alma, Delita, and herself.