She's deeply insecure (about her looks), drugged up (painkillers + something else?), emotionally damaged (divorce + dead of son), strangely submissive towards her domineering son, and probably just beep tired of it all to top it off.
I think she just wanted to put her foot down at the very start of the movie after entertaining Elias' delusions for a while, he needed to get over the dead of Lukas and finally face reality. Unfortunately she handled it pretty badly and things kinda' derailed from there.

The kid was suffering from paranoid schizophrenia. You don't talk somebody out of that.

Because they need it to stretch the film for 90 minutes. Seriously all she needed to to was sit down and have a conversation with her son "Your brother is dead, but I'm still here for you. You need to accept it and let him go" But no, let's be cryptic as beep and beat around the bush for days until the kid snaps and burns down the house.
First off, she needed to get her son psychiatric help. But also, the cryptic stuff is because she herself is beep in the head after Lukas' death and the divorce. Not defending it, but explaining it.
The kid(s) are always asking where there their mom went. Well the fun loving mom they remember "died" the moment Lukas actually died.

Because they need it to stretch the film for 90 minutes. Seriously all she needed to to was sit down and have a conversation with her son "Your brother is dead, but I'm still here for you. You need to accept it and let him go" But no, let's be cryptic as beep and beat around the bush for days until the kid snaps and burns down the house.

The mother wasn't in any accident
Fine, fine, I'll leave! But first I'm going to bother these peanuts! Hmm? Yes? Hmm? HMM?

Traumatic brain injury from MVA in which Lukas died, as well.

That was the kids imagination. Remember they cut the scene to him waking up, presumably dreaming it.
Waaarriors, come out to playy-y-ayyy!

What about the scene in the woods? When she stripped down and was twitching all crazy

One son dies (drowns), she and her husband separate, and she's a tv presenter who thinks plastic surgery will help her career, maybe her whole life. She's on quite a lot of meds (dropped under the bed, remember?) so how many are pills for post surgical pain? Those would keep anyone feeling flat, sleeping all day and night, and fairly non-responsive. Also, what mother could easily handle her only child not knowing his own mother?
Lukas is consistently the slightly taller twin, and is shown burning a bug with a magnifier. I think there are plenty of signs to show he's not the "good twin," but, Elias always carries out the acts "Lukas" suggests. Just as he likely spoke for his brother in life, it's common for one twin to be shy and the other to speak up for both. But why does Elias so readily go along with his twin's torturous ideas?
I just watched the film again and a second viewing, knowing the plot, is pretty amazing. In the end, when the mom says Elias isn't responsible for Lukas having died, I wondered if many be Elias really WAS responsible - did Lukas hurt him in life, like he seemed to want to hurt his mother so badly? Or is it all coming from Elias? Elias is more than a little twisted, and we don't know if Lukas is the bad twin after all, he could just be using Lukas as an excuse to act badly.

Her son died, she got a divorce and her other son is crazy. Then there's the surgery. It probably hurts a lot and why did she get surgery again? If she was in an accident, there's also that.