Answer: Hello there!
Here we want to complete the sentence "Carmen hola manuel, te llamo para invitarte a un pequeño evento. (1) está haciendo una fiesta para celebrar el aniversario de mis padres."
Wich can be translated to:
"Carmen, Hi Manuel... I called to invite you to a smal event, ____ it's doing a party to celebrate the aniversary of mi parents"
Here the answer can be the person that is organizing the party; for example the sister of the brother, and sister is translated to "hermana" in spanish, so the whole phrase can be:
"Here we want to complete the sentence "Carmen hola manuel, te llamo para invitarte a un pequeño evento. Mi hermana está haciendo una fiesta para celebrar el aniversario de mis padres."