Epilepsy can be pretty well controlled these days Verity - Epilin seems to be the drug of choice, at least in adults, and it seems to work pretty well.
Diabetes is a bit more difficult to manage I'm afraid - if she's Type 1 then she's insulin-dependent (i.e. her pancreas produces no insulin at all), and she'll require daily insulin injections - usually before every meal or sometimes just twice daily if she's lucky - and regular fingertip-pricking for blood glucose monitoring. At least being younger then she'll pick it all up easily and get used to it in time though.
Good luck to her though - there's no reason she can't lead a normal life :)