As events unfold, Ms. Alston chose to respond to this attack, while it was underway, by resorting to deadly defensive force. From another of the news stories linked above:

Police said after the women assaulted Alston and pulled her hair, she reached into her purse and pulled a handgun.

Having resorted to deadly defensive force, Ms. Alston will now need to be able to argue that she had a reasonable perception that she was defending against a deadly force threat. Is there evidence from which such a perception can be reasonably inferred?

Having resorted to deadly defensive force, Ms. Alston will now need to be able to argue that she had a reasonable perception that she was defending against a deadly force threat.

Given that the prosecution need be able to effectively attack only any one of the required elements of a self-defense claim, this vulnerability on the element of imminence would seem sufficient to doom any claim by Ms. Alston of self-defense.

Under Pennsylvania law a person using deadly defensive force does have a generalized legal duty to retreat if safely possible before the use of that force is lawful.