After he discovers who killed his mother while in rehab, the son of a Mafia Don must keep his brothers from discovering his forbidden relationship with a cop as they hatch a revenge plot against the rival gangsters who killed their mother.

Agree.

>> keep his brothers from discovering

I first read the logline thinking that the brothers are not involved and the protag and cop are the ones plotting! So it's all the sons in the plot, but the protag son has to keep a secret about the relationship? Does the protag also keep the plot a secret from her? Not exactly clear, seems she may be okay with it. It's also not clear if the son or brothers are actually gangsters or adopting the family tradition in this instance.? If they are gangsters, why should we care about any of this? Specify how many brothers.

>> as they hatch a revenge plot

Paint more of the picture and make it active. How long is the hatching and what happens after? What is the specific plot, at least at first, and what is the difficulty in pulling it off?

Scrap the whole subplot about the forbidden relationship (in the logline). It's superfluous to the primary story. The fact that he's in rehab is pretty irrelevant too. I'd also argue that the fact that he's the son of a mafia don is possibly not as important as learning more about who this guy really is. I get that's where the rehab bit comes in but there's nothing in here that tells me how this impacts the rest of the story.

What this boils down to is a very simple revenge film - not dissimilar to The Godfather in a lot of ways. This isn't a bad thing but I can't help but think this story needs more originality. Something we haven't seen. Rival gangsters at war and lovers on different sides of the tracks... it's very familiar.