Should I mention this to the mom? (9 yo says she wants to poison 6 yo sis)

Yes I have.

When I was young, my cousin was young, such as 8 years old or something, and she had said something like “I’ll kill you” to the neighbour’s kid…

So the neighbours had the police go to the girl and talk to her.

I did, though I sort of just dismissed it. I popped in and said: “Don’t say things like that, even if you’re angry, she’s your sister…” Then the friend said something else and we got side tracked.

I mentioned it to the parents, They kind of of hand waved it, but said they would move some pills (mostly head ache pills) from the bathroom cabinet to a more secure place in the bedroom.

I’ll be honest here… When I was about 5,if I had known that I COULD kill my baby brother I would have. No,I didn’t have a plan exactly,but I thought about holding a pillow over his face.
I didn’t,brother is alive if not well (cancer,diabetes,lost a leg)
Yes,I think this may be a problem. Have you mentioned to the girl how very sad Mom would be?

Mom used to tell a story about me, but I don’t personally have any memory of it. Seems I said to her once, at about age 6 or so, “I’m gonna kill myself.” Bemused, she asked me how I thought I was going to do that. Supposedly, I told her I was going to drink the ant poison under the kitchen sink.

She says that the ant poison and any other caustic substances were hidden elsewhere later that day and the kitchen cabinets fitted with those childproof whatchamacallits.

Seeing as I’ve been plagued with clinical depression with anxiety issues for pretty much all my life, this story doesn’t seem so far-fetched. The moral of the story is that some kids will conceive of it for whatever reason, and a percentage of those will actually follow through.

I would say something, just to be on the safe side.

I can see how a kid might say something like that in a moment of anger, or because she heard it from someone else. It was probably the plot on several TV shows at different times, too. I would be shocked if she was even halfway serious about following through.

However, prescription drugs are a very frequent source of drug abuse in kids. And maybe the girl doesn’t fully grasp the implication of putting drugs in her sister’s food - does she see this as something that might be funny, or does she really understand the seriousness? In any case, I think the parents should have a discussion about drugs, and make sure that the drugs are well controlled. These are rational steps for any parent to take, regardless of any death threats against a sister.