Final answer:
The subject pronoun you would use is 'they' (ellas) to talk about your friend Leticia and her sister.
Step-by-step explanation:
The subject pronoun that you would use to talk about your friend Leticia and her sister is they. In Spanish, the subject pronoun for a group of people is ellos or ellas. Since Leticia and her sister are both female, the correct subject pronoun to use would be ellas.
Learn more about Subject pronouns in Spanish