There are two versions of visas for under-18 visitors to the UK. The one you have used before is intended for travel with an adult, typically a parent.
There is another form for unaccompanied minors, intended for an under-18 traveling, for example, to visit a relative. You will need to prove you have somewhere suitable to stay, provide specific information about your sister, and have written consent from your parents. See Travelling alone. With your parents' consent your sister will be, in effect, a temporary guardian while you are in the UK.
As was the case for the accompanied visa, you will need to show you can support yourself during your time in the UK and return home without working or public funds.