Find my sister a car.

[Kezzieboy]

If she is pottering around cornwall and doing a couple of long journeys a year i'd just buy a petrol. Less to worry about reliability wise and you'll get a nicer car for the money. Mk6 fiesta 1.4 zetec would do the job perfectly.

Her driving will be half in Birmingham, half in Cornwall (where one doesn't really potter as everything is about 20 miles away) and then the journey between the two 6 times a year minimum, probably more like ten. 

What is there on a diesel that should be worried about? Modern petrol are just as complex, unless you by an inefficient old PFI. 

IMO the petrol/diesel argument is completely invalid with a £1500 budget.

OP hasn't even asked for opinions on whether she should get a petrol or diesel.

Implicitly I have, actually, I assume diesel is a better choice, but am happy to be proven wrong. 

Why not just ask her which car she wants and then get her the best/most reliable variant of it?

Because she doesn't know. Like 90% of the motoring population she doesn't care really, just wants it to be cheap and reliable.