Police arrested them on Tuesday as they were trying to sell properties that belonged to a resident who died at the home in the former fishing village of Santa Pola near Alicante, a police spokesman sad.
The family had also kept a car that belonged to another resident who passed away at the care home, set in a five-room villa in the resort.
Acting on a tipoff, police arrested the couple, both aged 59, as well as their 25-year-old daughter, after monitoring them for weeks. Their names have been withheld.
At the time of the arrest four pensioners were still living in the house.
The alleged role of the daughter was to persuade elderly clients of the Santa Pola pharmacy where she worked to move into the care home.
“She would show interest in them and establish an emotional tie with them. She would get information of how they lived, about their purchasing power,” the spokesman said.
The authorities said she targeted prosperous British retirees who only spoke English and had little or no family nearby.
Once they moved to the home the pensioners would be asked to hand over deeds to their properties and pressured into bequeathing their estate to them when they died, a police statement said.
Police said there were no signs that the residents were mistreated although their mobile phones were taken away.
Residents, who were monitored by cameras round the clock, would pay 2,500-3,000 euros ($2,800-3,400) per month to stay at the care home.
The authorities suspect the family had run the place for at least three years.
The suspects have been charged with fraud, membership of a criminal group and unlawful assumption of property.
Officially, just over 283,000 Britons are registered as residing in Spain, but by some estimates 800,000 to a million live in the country — working or retired.