I'm gonna guess that the event is a wake. The father died and is in his coffin in a suit. Mother was cooking the food for the guests who would soon be arriving, hence the apron. When the son came downstairs in his own suit the mother mistook him for her dead husband in the gloom of the parlor. She keeled over from shock and died. The son was so grief-stricken at losing both parents and also to have been part of the cause that he killed himself.
Wrong. I don't think you have enough information to make an educated guess yet, unless you recognize the source of the riddle (which, remember, isn't originally a riddle); and most of you probably aren't familiar with it.
Are we in some kind of fantasy setting?
Was it a family of elves? dwarves? fairies?
Aha! But here it gets a little tricky. To me, for example, elves and fairies are usually the same thing, and in some folklore (e.g. Icelandic) dwarfs and fairies are interchangeable. Issues like these are relevant here. I'm going to stick to the common modern understanding, however.
Case #3A family of three are found dead. Two are dressed in their finest clothes, and the third is wearing an apron.
What happened?Yes 
Did at least two of them die from the same reason? (more or less)Was it a barbecue party that had gone bad? (not exactly barbecue, and the party didn't have a chance to start, but close enough; let's say a feast)Are we in some kind of fantasy setting?
Were two of them married? (presumably)
Father and mother?
Was there a child? (in the sense of offspring)
Is cannibalism involved?No 
Are all three humans?
Is a help to suicide involved?
Was at least one a human?
Did one kill the other two?
Did at least one die of natural causes?
Did the one with the apron die of starvation?
Is at least one of them suffocated?
Did at least one of them die by poison?
Is at least one of them an animal? (debatable, but not what you'd usually call an animal, anyway)Does the triad consist of artificial intelligence?
Are the three in a funeral dynasty / crematorium?
Did at least one die from electroschock?
Were the two well-dressed bodies inflatable dolls?
Are any of them food products?
Was it a family of elves?
... dwarves?
... fairies?
Does the triad consist of aliens?
Are the three unicellular beings?
Did they [all] die at the same time?
Were the two well-dressed bodies in caskets?
Had [the other invited guests] already arrived when the three died?Irrelevant
Were other people (except for the three victims) invited to the party? (yes, but they don't come into the story; also, "people")