My two cats are sisters. They are three years old and one goes to the toilet outside but the other one always uses the litter tray. How do I get her to start going outside too?

Do your cats have the ability to go outside at any time they please?
If so, you could attempt to remove the litterbox. However, this may end up causing her to go where it once was.
It can be very hard to untrain a cat to use the litterbox so she will go outside, without having accidents in the house.

Thank you. Yes, they have a cat flap. I usually lock it at night but if I think I can get her to start going to the toilet outside I could leave it open. So you are suggesting just removing the tray immediately?

It's risky in the sense that she may go where the litterbox was, but removing it entirely may be the only option to get her to go outside instead.

Well thanks for your help. I think I may as well give it a go.