The answer can be two:
- tienen - Miguel y Juan tíos.
- son - Miguel y Juan son tíos
The option 1. means that Miguel and Juan have uncles and the option 2. means that they are uncles. For the first option, we use tienen which is the conjugation of the verb tener for the third person plural in the simple present. On the other hand, the word son comes from the verb ser and is the conjugation of this verb for the third person plural in the simple present.