Thanks for all the supportive comments guys. Much appreciated. It is what it is. Both of them have come to terms with the situation it seems (despite Peter having dementia it is well controlled and he fully understands what is going on), and my sister said last night it is all about the quality of what life he has left now which I think is the only way you can face this and retain your sanity.
My nephew (who I get on with really well) is visiting from Poland sometime next month so I may plan a trip up there to coincide with his visit. Ambleside is not the easiest place to get to from Cornwall but if my nephew is flying into Manchester I may arrange to meet them there and then get the train with them.