When were the parents of triplets D killed?

[Optimism is my Phil-osophy]

According to Dewey D. they were killed in a terrible fire about a year (or slightly less) after the Schism began on the triplets' 5th anniversary. (TPP Chapter 8) Here's the riddle:
Olaf claims to be responsible for the death of the other D family members, leaving only the twin brothers alive. (TPP chapter 9. When Olaf said this, he did not know of Dewey's existence.)
However, after the Great Schism, it is evident that Olaf stayed on the fire-extinguishing side. This is evident in TBL, where O. evidently refers to Olaf. This is also evident in ATWQ, where Olaf is mentioned as being on a mission with Beatrice. By this time the Great Schism had already happened, as Kit states that when she was about 4 years old the Schism began.

This consideration leads us to some possibilities.
The first possibility is that Olaf was responsible for the death of some members of the D family, even though he was on the fire extinguishing side at the time. This possibility opens up other possibilities. Olaf may have accidentally caused this fire. Another possibility, which I believe, Olaf intensively caused this fire and he was already an arsonist at that time. However, he pretended to be a supporter member of the fire extinguishing side. The possibility of VFD infiltrated limbs on the opposite side is real and very present in TBL.

The other possibility is something less exciting, but I need to talk about it.. Perhaps the Schism that Dewey is referring to when talking about the time of his parents' death is not the Great Schism. In this case, he would be referring to Olaf's Schism, which happened at the time Lemony was about to get married. Of course this brings us to a small but resolvable problem. By the time Lemony was about to get married, he was in his early twenties. Kit was older. So when Kit was about 25 years old, Dewey would be about 5 years old. Dewey would have to be 20 years younger than Kit. At the time of ASOUE's major events, Kit would be at least 15 years older than at the time of Lemony's marriage cancellation. (Violet had not yet been born at the time of Lemony's marriage cancellation). This means that Kit would be about 40 years old or older at the time of ASOUE's major events, and Dewey would be 20 years old. Dewey and Kit falling in love with this age difference may seem unlikely, but still possible. And of course, a woman in her early 40s can still be a mother. It would be a risky pregnancy, but it is still possible.

So which of these possibilities do you think is most likely? Is there any that I didn't see?

[Dante]

I think your first possibilities are probably closer to the mark, and are the conclusion I came to in interpreting these details; Olaf wasn't responsible for the deaths of the Denouement parents, but rather for those of other family members we have no awareness of. (He may not even be correct about this, but it doesn't really matter.) I also don't think it's unreasonable to propose that Olaf was secretly committing acts of villainy before he came out against the organisation openly; this appears to be implied by the Building Committee transcript in the U.A., in which he figures specifically as having "betrayed" V.F.D., but is at first treated politely by the other volunteers rather than as a personal danger.

[Hermes]

First, I don't think what D. says needs to mean the fire took place nearly a year after the schism. I take it you're reading 'I was four years old at the time' to mean he was just four; but it doesn't need to be read that way; if it was the eve of his fifth birthday he was still four. I've always supposed that he is actually describing the start of the schism here; the Denouement fire was the first overt act of the villainous side, and both the Denouement and the Snicket children were recruited at this point to keep them out of the hands of villains.

I agree with Dante that this means Olaf cannot have been responsible, both because he was on the firefighting side quite a bit later than this, and because he was a child at the time. (OK, he could have started the fire in the way Sunny does, but she had help.) He clearly belongs to the same broad age-cohort as Lemony et al, as he was in school with them, though he may not have been exactly the same age: some people have speculated that one's level in VFD education depends on when one was recruited, and it's likely that O was recruited at a rather older age than the Snickets, if he was previously at Wade Academy.

As for Kit's age, she can't be much under forty on any reading. L's engagement to Beatrice was at least fifteen years ago, probably more (depending on whether you believe the Violet Theory); if he was over eighteen at the time, that makes him mid-thirties now, and K is, as you say, older than him.

[Dante]

Following on from Hermes's point about Dewey's exact age - and I agree that Dewey is clearly indicating that the date of the schism and the date of his home's destruction are one and the same - Dewey's statement that "I was four years old when the schism began" (p. 179) is clearly intended as a direct parallel to Kit's "I was four years old when everything changed. Our organization shattered" (p. 27). Dewey's discussion of his childhood is actually a direct indication that he is more or less the same age as Kit - not decades apart.

[Optimism is my Phil-osophy]

So after these considerations, it is likely that Olaf killed other members of Dewey's family after Olaf's Schism. In this scenario, both Dewey and Kit have the same age range.

[counto]

According to the original book info, the Schism happened around the time of the Denouement triplet's 5th birthday. Which was also sometime after Jacques and Kit Snicket were born along with Lemony Snicket. It's implied Olaf burned down the Denouement family house down.

I don't know if this true or not. Because Olaf is also blamed for setting fire to the Baudelaire's and Ishmael's house when he claims he never did.

I think he was generally telling the truth about this.

[Optimism is my Phil-osophy]

After several considerations, we came to the conclusion that Olaf only started burning things when Lemony was already close to getting married. So Olaf did not become an arsonist during the Great Schism in the childhood of the D brothers. Despite this, Olaf may have killed other family members of the D brothers after he became an arsonist. Regarding the fire at the Baudelaire mansion, at the moment I defend the idea that it was an accident that Lemony was wrongly guilty of. Nothing in the books seems to indicate that Olaf had any control over what would happen to the survivors of that fire.