Why was Kestra Troi played by two actresses?

In the Next Generation episode "Dark Pages" the viewer learns that Deanna Troi had an older sister named Kestra. During the memory sequence when Lwaxana remembers how Kestra died Kestra is played by Kirsten Dunst. However, just a few moments later when Lwaxana comes to term with Kestra's death, Kestra is portrayed by Andreana Weiner.

It makes sense that the first time Kestra appears in Lwaxana's mind, in the Enterprise setting, that she would appear as Kirsten/Hedril, since the character Hedril was what had triggered Lwaxana's recollection of Kestra to begin with. But during the actual memory of Kestra's death this switch doesn't make a lot of sense.

Why was Kestra played by Kirsten in the picnic setting portraying her death?

Kirsten Dunst as Kestra at the picnic before her death

Andreana Weiner as Kestra