Well it’s their property and they are free to do as they please with it.
Having spent a large part of their income during their life on buying the house, why not realise some of that value, and spend the money on enjoying themselves in their later years?
On the downside, equity release firms typically charge an interest rate of around 5% per annum (which is quite high, given that it is impossible to default on the loan).
If you consider that both your parents will live for another 15 years – then their equity loan will have doubled over that period from £25k to £50k (based on a 5% PA interest rate). So if house prices don’t rise over the next 15 years (very unlikely), the remaining equity left to share between your sister and you would be circa £270k (I have not included any fees that might be applicable in setting up the equity release).
If I was your parents, I’d be tempted to release £50k (£25k won’t buy many luxuries these days) – but I also might prefer to downsize my property – buying something costing around £250k, leaving myself around £60k to spend after moving costs; thereby continuing to own 100% of the value of my home.