Answer: Vive
Explanation: vivo is when talking about where you live or if something is alive
Example: I live in Mexico - Yo vivo en Mexico
Viven is when talking about where multiple people live using the word they but in this case your talking about only on person (the sister of your cousins
Example: They live in Mexico - Ellos/Ellas viven en Mexico
Vives is a word used when asking someone where they live
Example: Where do you live? - Donde vives?
There’s more to it but these are the simplest explanations