What's the difference between filling a house and keeping a mistress?

There are several explanations for filling the house: 1. It used to mean that a woman married a man whose wife died. 2. Always refers to the wife who got married after her death.

People who fill houses are generally widows who are older or widowed for many years. But now the concept has gradually changed, and there have been many remarriages that are disproportionate to age. Of course, the most common way to fill a family heirloom is: if the son-in-law gets along well with his family, his parents-in-law don't want to lose him and are willing to stay in this family. Usually his wife's sister, the second daughter of his parents-in-law, will marry the original son-in-law. Of course, the premise is that both parties are willing.

The status of concubines is not as good as that of filling houses, because they are not the first houses, so they are generally under the jurisdiction of filling houses and are in a secondary position in marriage. Moreover, my concubine is not as good as a full house, which can be said to be backward in all directions.