If you think it's the housekeeper a number of things don't add up, I've listed a few of them below. Also @DK: What's your opinion on the letter? Who d'you think sent it and why?
Another thing I'm interested in is who was the real daughter of the first dead guy, it could be either of them or even both. I think that at least the maid is 'cause she tried to change the subject when Hattori asked her about it. However, that could be part of another possible motive.
Stuff that doesn't add up in the 'housekeeper is the culprit' solution
-The crying, why does she seem so upset over the deaths? You could say she's just good at acting, but there's been no foreshadowing of that at all which doesn't fit with how Aoyama usually does cases.
-The tea, why would she offer the tea to others when it could implicate her. Remember that Hattori and Conan only figured out the evidence for the culprit when the tea was given out, the maid didn't open hers which could suggest she has poison on her hands but like I said before: Why implicate yourself? The only other strange thing about the time with the tea was the secretary not realising her train was gone, despite the fact that she'd previously been wearing her watch and checked the time with it so it couldn't be broken. Chekhov's explanation for this (the cyanide reacted with the bronze) works the best IMO.
-If the company president always talked about how good Hattori was at deducting around the house then it'd make no sense to try and kill people while in his company.
-Also, the maid pointed towards optical illusions by showing Hattori the bathroom, what's the reason for leading them to your murder method intentionally? Really, that'd be pretty stupid and from what we've seen of her so far she's fairly sharp.
@Akakata:
That's 'cause in the one where the son was killed only those two could have arranged the cake into the right positions for the optical illusion (the piece further away looking bigger) to work.