On the eve of their 50th wedding anniversary dinner, Uncles Mark and Robert, help their family come to terms with alcoholism, an issue threatening to destroy a beloved nephew.

Hi Mark, in loglines we usually omit character names:
On the eve of his gay uncle?s anniversary dinner, a troubled man must again battle with alcohol and the impact it continues to have on the family; all the while celebrating enduring love.
my two cents for you

Okay.....
Is this more of what you are wanting to see/read?

On the eve of his gay uncle?s anniversary dinner, Marcus must again battle with alcohol and the impact it continues to have on the family; all the while celebrating enduring love.

Still confused. "On the eve of their 50th wedding anniversary dinner" refers to who?

And who is the protagonist?

Excellent! Thank you for your help and advise

As long as you keep the word count 25-35 words, including these extra bits of info helps clarify the story details. It wont be a synopsis. It needs to imply what happens in the beginning, middle and end (but no spoilers).

Who is the main character? Is it the nephew or one of the uncles? The best loglines tend to have one main character who drive the plot.

Really? It's not implied that the uncles are the ones celebrating and that they being two men are in fact a Gay couple? Is it necessary to point out such an obvious detail?