The Mistletoe Farm

The Mistletoe Farm series consists of two books by children's author Enid Blyton.
The first book of the series, "Six Cousins at Mistletoe Farm", was published in November 1948 and illustrated by Peter Biegel. The sequel, "Six Cousins Again", which shares the same characters as its predecessor, was published in 1950 and illustrated by Maurice Tulloch.
Plot summary.
The Longfield family, who are 15-year-old twins Jane and Jack, 11-year-old Susan and Crackers the spaniel, live at Mistletoe Farm with their parents Mr. and Mrs. Longfield.
When they receive a phone call from their uncle who informs the family that his uninsured townhouse has burnt down, they are asked to look after his three children, who are 16-year-old Cyril, 14-year-old Melisande and 10 or 11-year old Roderick. The Longfield children are initially alarmed of the prospect of their three spoilt cousins from the city living with them, and at first they find it difficult to get along with each other. But after a number of challenges and hardships affect Mistletoe Farm, the city kids learn to adapt to life in the country and get along with their cousins.
In the second book, Cyril, Melisande and Roderick have moved into a nearby, much more modern farm with their parents. Their mother, Rose, has difficulty adapting to the life of a farmer's wife, their father, David, has various setbacks, and the children have trouble settling down.