Final answer:
The correct verb form to complete the question is "vienen" which means "they come".
Step-by-step explanation:
The correct verb form to complete the question "¿Tus hermanos a la fiesta?" is "vienen".
In Spanish, when referring to third-person plural subjects (such as "tus hermanos" - your siblings), the correct verb form is "vienen" which means "they come".
For example: ¿Tus hermanos vienen a la fiesta? - Do your siblings come to the party?
Learn more about Spanish verbs