DD1 (nearly 5) had been playing up all evening. Asked for something specific for dinner then announced she didn't like it anymore when put in front of her, that sort of thing. Picking up her poorly toddler brother when asked not to as it upset him.
Bedtime and we all go upstairs. DD1 at the top and 21 mo DTs following. DD1 decides to create a "red line" not allowing the twins past the stair's head. I call up to her to stop, it's dangerous - she doesn't move. DTs rapidly approaching her with me just behind. I ask her twice more to move, now - she doesn't - DT2 now at top and collides with the "red line", toppling back down the stairs.
As I was right there I caught her , but it was heartstopping. Made sure DT2 okay then picked DD1 up, carried her to her room and really shouted at her. I try not to shout but I properly lost it. 
Feel so shit. DD1 in floods of tears. Argh.