Posting here because I know I fucked up but need to know how better I could have handled it.
DP was sitting at the PC with DD (3 months) on his knee and DS (3.5) standing next to him, and as I entered the room he started shouting at DS asking him what he'd done. DD had started crying obviously in pain.
I went straight over and asked DS what he did and he said he pinched her fingers hard. I saw red and pinched his finger, and then had a real go at him - it's very unkind, she's just a baby, why did he do it (because "sometimes she scratches or pinches" him) etc. I know that was a pointless, futile, unkind thing to do and I am truly ashamed of myself for doing it. So he started crying and rambling and I kept on at him and it was all bad, meanwhile DP's trying to soothe teary DD.
Afterwards DP had a go at me for butting in and not letting him handle it - I do have a history of this and I realise it is bloody annoying. On this occasion as DP was holding DD who was hurt it didn't occur to me not to intervene. So he's pissed off with me, probably fairly, but I didn't honestly think it would be a problem.
So we're in a mess. And it's my fault. As for why a 3 year old who's never, ever been anything but affectionate towards his baby sister would suddenly pinch her, I have no idea, and not a scooby about how to handle it.
:(