I feel your pain. Two rejections of my own this past week. At least they were personal notes, not form rejections. I believe they must have read my query!
One thing I changed in my own letter was to include a short sentence in bold that is the gist of the book. Something short, an attention getter that makes them want to read more.
Your story seems interesting with drama and emotion that make your characters interesting. I lose track of the story line, however, with all the negative adjectives. To provide feedback, I had to read it several times to make sure I had the gist of it. I doubt an agent will take the time. But I'm not an agent, so who really knows? Just my take on it.
Anyway, I would tighten considerably, spell the plot out rather than the emotions. My rewrite of the first paragraph would be something like this:
Ava, a London solicitor, searches for her missing jailbird father, Andrew. What she finds instead is Nick, an acrimonious and antagonistic half-brother of which she had no knowledge. Together they stumble along, discovering family secrets of murder and blackmail. Is her father the rogue she has been told, or is her domineering mother the one behind the lies and secrets?
The second paragraph can explain something about Ava's past and her self-discovery.
Just my thoughts but maybe they'll be of some use. Regardless, it sounds like a good read.