A young American executive with strong family bonds seeks revenge for her sister's murder only to be confronted with the realization that the American Revolution never ended and her family secretly fights for the British side.

.... I beg to differ: http://www.mojvideo.com/video-south-park-the-snuke/f7d32ae02bb77b2eb3c0

As Nir Shelter said. Particularly on the issue of plausibility. It doesn't seem plausible that anyone in the modern era would still be fighting, albeit covertly, the American revolution.

There are four separate and unrelated descriptions of the main character:
"?young?"
"?American?"
"?executive?"
"...with strong family bonds?"

Just as in a script, every word in a logline is imbued with a meaning through a semiotic system of linguistic conventions. As a person reads each description a new association is formed, in their mind, between the MC and a memory or understanding they have of the word used. As this character description is so lengthy and varied the end result is confusion as to the nature of the character. Best not to fragment the character's devices, tendencies and urges in a logline rather condense them, into a singular meaning that contributes to a conclusion that puts them at risk of achieving the main external journey's goal.

Secondly there appears to be no inciting incident, what is the starting point of the story? And why must she now weak revenge?

As a result of there being no clear starting point the time frame is not clear. Did the sister die a week prior to the start of the story? Or a year? Or many years?

For a main character "...to be confronted with the realisation that?" is not a strong enough action to fulfil the requirements of a cinematic medium. What is her objective outer journey goal (that results from the inciting incident) and how will she achieve it?

Lastly what period is the setting? Is this a few years after the American revolution or modern day? If this is modern day the fact that the American revolution is still on going in your story presents a huge plot hole that needs attending to. I for one would find it very hard to believe that any American would actually fight to return the USA to a British colony status in modern day America.

This is not a problem with probability rather plausibility that would prove a huge hurdle for most audience members to suspend their disbelief.

Hope this helps.