I've been looking for this really specific episode all year! No matter how many search terms I think of I can't find it. I'll do my best to describe it here.
So this is a much older episode. Most likely a dubbed episode so i'd say somewhere between episodes 80-130. (I may be wrong though.. I couldn't find it searching them one by one).
The plot:
An older mother close to death resides on a property that means a lot to her. She has three kids, two of which want to take the property for themselves. If I remember correctly the son wanted to build a shopping center on the property and the daughter just wanted to sell. The other daughter lived with the mother and took care of her and had no interest in the property. So, naturally, the mother would like it to go to her.
Conan and them show up to the house and everyone is offered tea or coffee. Eventually the son passes out due to poison that was put in his cup or drink.
Fast forward.
Conan realizes that the older woman knew that the cup was poisoned to kill her and intentionally switched the cup to give to her son without him knowing. One of the daughters may have been the culprit, I don't remember.
What I found strange about this episode was that it was the only one Conan didn't call out the murderer completely. He knew that although the woman was the target, she switched the cup knowing what would happen to her son. On top of that, Conan pulls her aside and tells her that he knows why she doesn't want to give up the property, and that there are bodies buried underground that she had put there long ago (it was a sentimental side story that I forgot..) After that, she thanks Conan and comes to the realization that he's more than just a kid. Then they leave.
Thanks for your help! Can't find this one for the life of me.