Speculating on who actually poisoned Jade
Feel free to just leave the character's name if you want.
If you feel compelled to explain why in detail, all the more better.
I have my own assumption and it isn't anyone I've come across yet in any of the other topics discussing episode 5.
I'm going to give my own answer regarding who I think poisoned her and hide it so that it doesn't influence your own answer in any respect. Well, at least, before you answer this to begin with anyway. Once you've responded with your own answer, feel free to view my answer at that point and we can discuss if you think I'm completely off my knockers or my guess is as good as anyone else's or quite possibly I've nailed it.
I'm thinking it's the lady who was hiding under the table with the child when Sunny and his Baron came back after attempting to hunt down the Widow. Given somewhat of her backstory and knowledge and then the Baron's response at the end of their scene, "thank you for your loyalty." She may be responsible for the Baron's original 2nd wife's death actually as well.
Like they say, time will tell.