I think you need 2 or 3 dry runs beforehand, increasing the time away and the level of responsibility each time.
The 40 minutes away, by train, would be the biggest issue. Is there a close family member or friend who is much closer who could be their emergency contact.
You also need to lay down some rules.
- Who is in charge of what?
- Who to call in case of emergency?
- How to deal with x, y, z?
Also
The 12y needs to know that the 14y is in charge.
The 14y needs to know they must act in a responsible manner and not do anything to deliberate antagonise the 12y.
I also wouldn't really call in babysitting as such and wouldn't be paying the 14y.
At 14y I was regularly babysitting my younger cousins until midnight and later. I was changing nappies, putting them to bed, feeding them ,etc. Likewise I would look after my own sister who is 9/10 years younger (plus we have a brother a year young than me).
A lot depends on the individual children involved, but some practise runs would be a good idea.