[Joshua]

A lot of evidence would be needed to convince the inspector that you became a beneficial (part) owner in the property at six years ago and that an implied trust was created at that time. Did you declare the rent each year? If we were wearing the Inspector's hat we probably would look at this with a large dollop of skepticism.

Anyhow, when your sister sells or gifts the (rest?) of the property to you then she will have made a disposal for CGT purposes and you may also have to consider any stamp duty payable on the transfer.
Thanks,
Josua