DD has been OK with DS really, apart from the odd bit of jealousy, and not wanting to share toys etc etc

She is usually fine to be left with DS for short periods (when I nip to loo, make a drink etc). ALthough she has on occasion pushed him over from a sitting position.

However, I was shocked when I left the bathroom tonight to fetch a towel to return to find DD throttling DS- she looked like she was enjoying it, and his face was all red and he's got finger marks round his neck as she was doing it hard.

We asked if she'd done it before (she said no) or whether it's something someone had done at nursery (she said no) so it was really unexpected and disturbing.

Do you think it was a one-off "experiment" or should I be more worried about this?

Of course, I will make sure I don't leave them for a while by taking one of them with me and watching for any more signs of violence, but i don't know what else I can do? I know I didn't get on with my little brother and was quite mean to him but I was older and knew I was doing wrong.

Dd was similar at the same age. I would do as you are doing- not leave the two alone together- but not worry about it further. Dd has grown into a loving sister since and has no problems with violence whatsoever.

Basically, 3yos have very little impulse control. And, of course, no understanding of the implications of strangling somebody. So don't read too much into it. But don't give it a chance to happen again. We put a lock on ds's door at this time, so I could go to the loo or answer the phone. But then he was younger than your ds.

Thanks for the reassurance cory.

I will keep my beady eye on things, and make sure they are safe when I have to leave them for a moment.

I had read somewhere not to leave under 5s with a baby, but I had got used to trusting DD as she seemed so good with him lately.

DD is 2.9 and DS 5 months. DD is fine most of the time but every now and then seems to enjoy inflicting a bit of pain on DS. Either because she is just over-tired and being over-zealous or sometimes attention-seeking I think (when feeding DS etc).

My solution was to buy a play-pen about 2 months ago (didn't have one for DD)and it's been fantastic. Means I can leave the 2 of them in a room together (DS in play-pen) whilst doing chores and not have to keep checking every second (although obviously wouldn't have helped with the bathroom scenario).

I think it's exactly as Cory says - 2/3 yr olds have all the impulses but none of the empathy or willpower that keep them in check. It's reassuring to see that DS usually has lots of smiles for his sister regardless and doesn't seem too psychologically scarred .