Final answer:
The subject pronoun to use to talk about Leticia and her sister is 'ellAs'.
Step-by-step explanation:
The subject pronoun that you would use to talk about your friend Leticia and her sister is ellAs. In Spanish, when referring to a group of females, the pronoun used is ellas. So, in this case, you would use ellAs to talk about Leticia and her sister.
Learn more about Use of subject pronouns in Spanish