Well, their father was a good man. Good, good man: goes back into a burning building to try to save people good.
What if that wasn't an accident?
What if Mrs. Darknoll visited her husband at work not long before the accident? 
What if she ...did something that caused it, knowing her husband's response?
What if she preferred the idea of being a widow.
What if she's where Bram, and to a lesser extent Gideon, gets it from?
and that, really, Gideon has more of his mother's spirit to him than Bram does. He was raised with all these platitudes of goodness and kindness, but... well, ultimately, he is the sort to meddle and manipulate "for your own good."
Whereas Bram is actually much more like his father. Who cares what the laws say, or what I SHOULD be doing? This is what I NEED to do.
Gideon's Lawful Evil but good at hiding it.
Bram's ...I'm not sure but it ain't lawful. Nor evil. Chaotic Good?

CG struggling to be CE?

CG trying to be LE or True Evil.
Compared to LE masquerading so successfully as LG that even he believes he is (Gideon)

and meanwhile I'm over here thinking, "Killing off her husband was obviously the most outlandish thing she's done, but I'm sure she's had a string of poisoned servants and boarders in her life." 
There for a while, right after the Accident, money was tight, so they took in boarders, and the ones she didn't like she'd poison. 
She's a serial killer but she's very rigidly controlled, which is unusual.

I wonder
If at any point she's considered poisoning Bram
He's ruining her image, after all

Mr. Darknoll wasn't advancing as quickly as she'd hoped he would. And he wanted more children. She was DONE with this 'giving birth' nonsense. Two boys was enough.
Oh yes, I'm sure if she'd been able to get Bram to come to dinner during his criminal days... well, it would have been for the best for the poor dear.
But Gideon, strangely, would never have allowed Bram to set foot in the house in the first place.

OHHHHHHHHHHH Gideon would have arrested Bram and taken him to jail, but Mama would have baked her poor wayward lad a special treat, at least. A last meal for him. Liberally poisoned.
So that way there wouldn't be a court hearing that would embarrass the family
And the only reason nothing's poisoned now is that not only has Bram turned his life around, but he's away from London. He can't reflect negatively on the family now.
Most of the Darknoll boarders and servants would have survived, so no one would think twice about the occasional...
OH MY GOD THE WALLPAPER
Mama has a lot of green wallpaper in the house. It was colored green with an arsenic mixture.
And by 1885, this was a KNOWN THEORY. There were still people who didn't believe it to be true, but it was well-known and widespread
Depending on what year it is, she should know better than to have that wallpaper in the house, BUT SHE STILL DOES.
Mama doesn't know about the toxic wallpaper... because Gideon hasn't told her.
It would distress her too much, the poor thing.
And, honestly, she's getting on in years.
It's better for her this way.
She'd hate to have to tear down all that paper, after all (though Gideon's removed it from his room)

I'm getting serious Dumbledore vibes
'For The Greater Good'
It's alright for bad things to happen, so long as it's for the Greater Good

So, yes, she doesn't eat as much. Yes, she sometimes has trouble breathing. She's forgetful, and her heart isn't what it used to be. But then, she's old. That's all normal for a woman of her advanced years. Nothing untoward AT ALL.
Gideon is very much a Greater Good type
by his definition of "Greater Good"
Gideon doesn't spend much time in the rest of the house. But then, he's always out working.
Though... at this point, Giddy might have to take a page from Mama's book (though I doubt he knows about any of her... activities)
Bram was, no doubt, removed from her will because of his reputation and lawlessness.
She might consider re-adding him.
And, sure, Bram's on the straight and narrow now, but he's obviously still a wastrel. He's still a schemer, even if he's scheming for good now, and he'd just... that money would be wasted on him.
Think of all the good Gideon could do if he were more comfortably set up in life.
So Gideon needs to be the one who gets all that money, still. 
Mama can't be allowed to make changes to her will, if she's considering it.

The other option is to expose (or frame) Bram for still being a ruffian
The arsenic wallpaper is slow, after all

well, if Bram's leaving soon, he won't have time for that.
can just poison her more directly.
too bad the house is electric now; a leak of illumination gas in her room would've been perfect.
She'd have gone to sleep and just not woken up
but all the best houses are electric now. they had to.
ah well