Hello there!
Answer: Tu te diviertes con tus hermanos.
Let's first consider, what is a 'reflective verb' anyways? -- A reflective verb is a verb that has a subject, directing it's self back to the subject. For example; " Bella put (her) shoes back on".
The first subject is Bella, and the second subject would be (her), which is Bella. Understand? A reflective verb is a verb that directs right back to it's self.
In the following sentence: Tu te diviertes con tus hermanos.
"Tu" ... which in English it would be translated as 'you' is the first subject.
"Tus" would be the reflective verb. Because it's directing back to the first subject.
Answer: Tu te diviertes con tus hermanos.
Hope this help!