Because Lwaxana's memory sequence was triggered by Hedril's (played by Kirsten Dunst) resemblance to Kestra. So while Lwaxana was in her coma, her Metaconscious used Hedril to represent Kestra. The memory sequence of Kestra's death was stil within Lwaxana's head, and Deanna did not yet know about Kestra, so Hedril's image suddenly changing to a strange girl certainly would be confusing. Also, in-world, Lwaxana had not revealed the truth until then.
From the Wikipedia entry for the "Dark Page" episode, pertinent parts bolded:
Lwaxana Troi returns to the Enterprise, this time as the teacher of an
alien race learning to speak. Their native form of communication is
telepathy, but they want to learn spoken language in order to interact
with other races. Lwaxana comes aboard with Maques and his young
daughter, Hedril, who is Lwaxana's star pupil.
Suspecting that Lwaxana is not her usual, flamboyant self, Deanna
tries to investigate. When Lwaxana starts showing signs of fatigue,
she is taken to Sick Bay to be examined. Beverly tells Lwaxana that
her interaction with the aliens is more taxing on her telepathic
glands than she is used to and is asked to refrain from telepathic
contact with them until she can recover. Deanna helps her mother by
verbally communicating with Maques and Hedril, but Lwaxana still uses
her telepathy when they don't grasp the verbal concepts. After an
incident where Hedril falls into a pool of water in the arboretum,
Lwaxana falls into a coma. With no signs of physical trauma, Crusher
deduces that her telepathic abilities have caused her mind to collapse
in on itself from the over use. Maques uses his ability to form a
bridge between Deanna and Lwaxana, and the former finds herself
walking through corridors like on the Enterprise. Lwaxana's mental
defenses pick up, first conjuring Deanna's father to distract her,
then an image of Picard himself ordering her to sever her connection.
She sees past these ruses and encounters Hedril petting a wolf, but
the girl does not seem to know who "Hedril" is. Lwaxana then charges
in screaming for Deanna to get away from her, causing Deanna to break
Deanna and Riker search through Lwaxana's things and find several
pieces of the puzzle missing; journal entries for a journal Lwaxana
kept since being married had blank pages from shortly after Deanna's
birth going on several years. The image of Hedril in her mother's mind
does not add up. She tries again to reach her mother, this time,
Deanna encounters the wolf by himself who chases after Deanna through
the deck until she encounters a Turbo Lift that opens into space.
Hearing her mother call for help, Deanna jumps in and lands in the
arboretum where Lwaxana had collapsed. Lwaxana is there, still begging
Deanna to leave, but Deanna refuses, stating that a repressed memory
is killing her and she must relive the memory in order to survive.
Suddenly, an image of a human girl who looks like Hedril is playing by
the water with a puppy, and Lwaxana calls her Kestra. Deanna urges
Lwaxana to relive what happened, and she tearfully remembers a tragedy
when Kestra ran after the puppy when it got away, though it was
unclear what had caused her death. Lwaxana's repressed memory of
Kestra and her resemblance to Hedril led to her coma. As Lwaxana
recalls happier memories of Kestra, Deanna tells her to share them so
she can learn about the sister she never knew. The women awaken in
Sick Bay holding hands.