My recently widowed mother has just sold her bungalow and is moving in with my sister and her family. My sister's property has a substantial interest-only mortgage attached, but this is still a minority of the property's value. If my mother was to buy a share in the property by using her house sale monies to pay off the mortgage, would there be any tax implications for any of the parties involved? The intention would be that after my mother has died, the property would be sold to release her share of the property's then value, so that her estate could be wrapped up.

There will be no capital gains tax for your sister because she is selling part of her principal private residence. There may be stamp duty land tax for your mother depending on whether she pays less or more than £125,000, and if more, how much she pays. However, after your mother has died, surely her share of the house would be inherited by her inheritors, and furthermore it seems to me rather improbable to sell a share of one's home to a third party, so I'm not sure what you mean in your last sentence.