My answer is based on the assumption that the property will never have been your main home by the time you sell it.
When you inherited the house, it was worth £90,000. Therefore, the base cost for your one-third share is £30,000 (£90,000 / 3). You then paid £30,000 to buy out a third party so that added £15,000 to your share of the base cost of £30,000 bringing it up to £45,000.
If you inherit your brother's half share, the cost to you will be the value of that half share when he dies. Assuming that you sell it shortly after your brother dies, let's assume for £130,000, it's safe to assume that his half share will be worth £65,000. That brings your cost up to £110,000 (£30,000 + £15,000 + £65,000).
You will make a gain of £20,000 (£130,000 - £110,000). Deduct the annual CGT exemption of £11,100 and you are left with a net taxable gain of £8,900 on which you will pay CGT at 18% which is £1,602.
I hope this helps but let me know if you have any further questions.