It is indeed strange that she should have been hacking into his emails, so it may be that there were some cracks in the relationship anyway - sustaining a relationship across the miles can be extremely difficult.
I can understand her feeling hurt and angry - do you know what the content of these emails was? I am assuming that it was in some was compromising and inappropriate. Are you completely sure that there has been no physical relationship? The degree of her anger would imply that she thinks there has been.
Since you will be walking a bit of a tightrope and not wishing to alienate anyone, especially at this stage when the facts are not fully clear, it may be best to reply that you can understand her feeling upset and angry and that it would be best to try and get a conversation with her OH to sort out what is really going on before talking to the children. It would be worth saying that you would be concerned that the children might be upset at this stage when no-one quite knows how things might be worked out between them; and that it might be better to talk to the children later when the situation is clearer.
How hard it is when we feel our children have not behaved as we might wish and we feel disappointed in them. But I do feel that the facts need to be fully established before anyone jumps to conclusions, for everyone's sake, and for the children first and foremost.
I am so sorry that you are having to face all this.