Formal:
* Je m'ennuie de mes frères. (This is the most literal translation and quite formal.)
* Je pense beaucoup à mes frères. (This means "I think a lot about my brothers," which implies missing them.)
Informal:
* J'ai le cafard sans mes frères. (This literally means "I'm down without my brothers," and is more casual.)
* Mes frères me manquent. (This is a more direct and personal way to say "I miss my brothers.")
* J'aimerais bien revoir mes frères. (This means "I would like to see my brothers again," which implies missing them.)
You could also add details about what you miss about your brothers to make your sentence more specific:
* Je m'ennuie de nos parties de jeux vidéo. (I miss our video game sessions.)
* J'ai le cafard sans les blagues de mon frère. (I'm down without my brother's jokes.)
Ultimately, the best way to say "I miss my brothers" in French depends on the context and your relationship with the person you're talking to.