Theory:

      And when she was old enough she married the King of Cambodia and was carried on a white elephant all the way to the city in which the King lived. But her sisters never slept with their windows open, so they grew extremely ugly as well as disagreeable, and when the time came to marry them off they were given away to the King’s Councillors with a pound of tea and a Siamese cat.
Explanation:
 
When September reached marriageable age, she married Cambodia's King. She was transported on a white elephant to the King's capital.
 
12.jpg
A white elephant
 
September's sisters, on the other hand, never slept with their windows open. 'Not sleeping with the windows open' is the metaphorical take on the Princesses' unwillingness to accept change. As a result, they were very ugly and unpleasant. When they reached marriageable age, they were offered to the King's councillors along with a pound of tea and a Siamese cat.
 
White elephants are common in Siamese culture. The Kings of Siam usually bestow someone with a white elephant as an honorary gift. White elephants are also considered sacred, in addition to being a status symbol. On the contrary, a pound of tea is a common complementary, from the farmers to the buyers, as a grateful appreciation for a high-priced purchase. While the white elephants are unique and one-of-their-kind, the Siamese cats are domestic animals, found almost in every household of Siam.
 
11.jpg
A Siamese cat
  
While September conquered her possessiveness of the bird and grew into a beautiful lady, her eight sisters never allowed for positive change. As a result, they grew up to be unattractive and awful. This comparison is again related to the proverb, "The face is the index of the mind." According to this context, the other eight princesses were greedy and manipulative. As a result, they grew up to be ugly and miserable.
 
In addition, September made a wise decision. As a result, she gained the affection of the better bird (the Nightingale) and a happier marriage (her life with the King of Cambodia). At the same time, the other eight princesses resisted good progress, spent most of their lives in a locked room with the mediocre parrots, and married the King's less-worthy councillors. The parrots and his councillors were identical since they repeated the same thing in seven different ways, none of which made sense, as the King had previously said.
 
Meanings of difficult words from the paragraphs:
 
S.No
Words
Meanings
1
DisagreeableUnpleasant \ Unattractive
Reference:
National Council of Educational Research and Training (2008). It so happened. Princess September - Somerset Maugham (pp. 34 - 44). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.