I have never heard anybody say or claim that if you live with someone, who has lost their spouse, the partne r(unmarried) gets the house when the widow/er dies. I havent even read it in the problems column of a magazine or newspaper.
There are people (not just women!) who think that if they have lived with someone for a significant number of years and, in particular if they have given up a career or had children by that partner they should be entitled to a share of the house, but that is entirely different
And, no Frank, children who own half a house cannot automatically move in and sell the house when the widow/er is still alive. If wills are properly drawn up what can and cant be done will be clearly set out. The owner of a holiday home where my parents lived left the home to his wife and two children. His second wife got £15,000 of the value and the children, from the first marriage the rest. By the time the owner died the property was worth over £100,000. The second wife was so agrieved that her share of the property was so small, she refused to sell her share. This meant the house could not be sold. The house stood empty and decaying for nearly 20 years until she died. By then it was in an unsafe condition and had to be demolished and the land sold as a housing site, for about £250,000!!