When their Uncle is murdered by the Irish Mafia because of their dead father's debt, an honest citizen joins his teenage brother to take their revenge. They must fight against mobsters and the corrupt policemen who control the streets of Boston to ultimately dismantle the oldest crime group in the United States or die trying.

Hello,

2 things

1- you should keep it one sentence.

2- you have to mention only the most relevant elements and be sure that all the elements are linked in a logic scheme.

So,
the reason Mafia kills the uncle is irrelevant. What is important is that he’s the uncle of the main character, this motivates him enough o take a revenge. A teenage brother? a big brother should protect his teenage brother and try to keep him away from the dirty business… no logic here.
You say “they must”: why? Can’t they just move? Or paying the debt? Or join the mafia?
You have to make a creative decision: is this a revenge movie? cut all teh rest. is-it a drama? fous on drama – just keep focused.