How much tax will my sister have to pay?

My sister bought a second property in 2002, with a lump sum left from her relative.  For the first four years it wasn't lived in and her son lived there from time to time. Since 2006 until now it has been rented out. She is coming to the end of her mortgage in two years’ time and cannot pay the remainder off as she has an interest only mortgage. She was not a high earner and worked up until three or four years ago.  She paid £218,000 for the property and has been given a valuation now of £500,000, but of course that doesn't mean to say that it will be sold for that. Can you give a rough idea on the amount of tax she will be liable for should she sell it?

Assuming a sale proceeds of £500,000, and a base cost of £218,000 (and, for simplification purposes, ignoring any allowable incidental costs and capital expenditure), the gain is £282,000. Assuming that the capital gains tax annual exemption is available, and assuming a value of £11,000 (for 2014/15), the taxable gain is £271,000. Although your sister may be a basic rate taxpayer, based on her other regular annual income, nevertheless the vast majority of this £271,000 will fall above the higher rate threshold, and will therefore be taxed at 28%. So she can expect a capital gains tax bill in the region of £75,000.