This seems pretty simple, and the only costs are a bit of legal and stamp duty, which are a small fraction of the CAT in issue.
Step 1: value the property in January
Step 2: On 1 February, gift your sibling a proportion of the property equal to the amount you want to gift her, based on that valuation (roughly 2/3 based on the figures in your OP).
Step 3: Sell the property. You'll get 1/3 of the proceeds, she'll get 2/3. Provided your valuation was reasonably accurate/ erred on the high side, she shouldn't owe any CGT on her share of the proceeds.
The tax fallout is that you've made two disposals, totalling the whole 500k and will owe CGT accordingly*. This won't be due for payment until late next year.
Your sister will have received a gift of property, but the CGT/CAT offset will apply to the amount in excess of the relevant threshold plus 3k, because your CGT liability on the disposal to her arises on the same event as her CAT liability.
(* Just in case anyone pedantic ambles by, yes, part disposal rules come into play, so the gain / CGT arising on the gift won't be exactly 2/3 of the total gain / CGT, but it shouldn't affect the overall outcome.)