If the siblings get on and are sensible then I wouldn't have a problem with this. I would worry, but I am a worrier, and worry every time DS goes on school trips/residentials etc but I know that this is my problem, and mustn't let it impact DS's life.
I would have been annoyed if I had not been involved with discussions about the trip.
However, I would have a problem with this if it was just the 12 year old doing it, and am surprised by the number of people who have said the 12 year old should be able to do it by themselves. To be fair I would find a journey with numerous changes daunting! So I am wondering whether people saying this live in areas where public transport is plentiful and regularly use the tube.
We live rurally, not the arse end of nowhere, but public transport is not great eg mainline railway station is about an hour's drive away and the bus service is not regular. So we tend to drive most places. DS(13) gets a bus to school but it is a designated school bus, just for his school. Can't remember when he went on a public bus (in fact can't remember when DH last went on a bus). I have tried to encourage him to get the bus with friends to go to the nearest main town, but he has never been interested, and to be fair by the time we have sorted out the logistics of getting everyone together from the various villages, it would be easier to drive!
I think DS has been to London 4 times, twice with school and I think they were taken by coach everywhere so wouldn't have used the tube. Last time we went as a family was 5 years ago, so he probably wouldn't remember how the tube system worked. He probably has been on a train twice since then, and that wouldn't have involved any changes. So he isn't really public transport savvy at all. And he isn't used to vast numbers of people milling around.
If DS had to do this journey alone, we would obviously discuss everything in detail and the 'what if' scenarios, but it would be a massive thing for him to do.