Madam Tan Teck Soon says, for 26 years, she has paid the Housing Board $277 each month - mortgage instalments for the three-room flat that she lives in.

She paid over $117,000, including upgrading costs and conservancy charges, said the 76-year-old canteen vendor, but she might soon have to leave her home. In March, she said, she learnt her granddaughters were trying to sell the flat.

To stop this, she has sued both Ms Michelle Ng Li Xuan, 26, and Ms Isabella Ng Su Xi, 25.

The case is pending in the High Court, and the two sides met for a pre-trial conference on Tuesday, said lawyer Chia Boon Teck, who is representing Madam Tan pro bono.

Both sisters are registered owners of the flat, which they inherited when their father died in 2009.

But Madam Tan said she had single-handedly paid for the flat since its purchase in 1990. Her granddaughters were only holding it in trust for her, she said. In her affidavit, she said they were trying to sell it and "swallow" the proceeds.

The 10th-storey flat in Bedok South was bought under the name of her son - the sisters' father, Mr Ng King Nguang - said Madam Tan, who was then registered as co-owner of another flat with her older son. The disputed flat has an estimated value of about $330,000 now.

"The flat was registered under Ng's sole name at that time with the understanding between Ng and me that I was the sole owner," she said, adding that she paid the initial sum of $20,000 for the down payment and renovations.

She was registered as an owner of the flat in 1992, after the other flat was sold. But seven years later, Mr Ng chalked up about $100,000 in debts, she said. He then purportedly asked her for help. She said he wanted her to sell him her share of the flat so he could get an HDB loan on the pretext of paying her.

She said she did not get any money from the sale, but lent him $61,000 instead. He used the entire sum to pay creditors, she said.

"I'm not a lawyer. I didn't understand the implications. My son and I understood the flat still belonged to me," she told The Straits Times.

In 1992, Mr Ng divorced his wife, who got custody of Michelle. Isabella, about one then, grew up in the flat with her father and Madam Tan.

In 2009, Mr Ng died after a heart attack. The sisters inherited the flat, along with his mortgage life insurance payout of $40,200.

"I did not understand how (they) could sell the flat and throw me onto the streets when I had paid for the flat entirely single-handedly," Madam Tan said in her affidavit.

Both sisters denied trying to sell the flat without her knowledge.

Ms Michelle Ng disputed that Madam Tan had made all payments for the flat. "What I understand is that my dad was the one doing the payments," she said, adding that she and her sister let Madam Tan live there as it was near Madam Tan's workplace.

Ms Ng said Madam Tan made some payments for the flat after Mr Ng died, but that was because Madam Tan was living there then.

Ms Ng said the flat was an asset passed down to both sisters by their father, which they should be able to sell, and they had offered Madam Tan an alternative place to live - with Ms Isabella Ng at her upcoming BTO flat in Choa Chu Kang.

Said Ms Michelle Ng, a former marketing executive: "I'm not working at the moment. I'm expecting my second child. I'm not taking the money to go and enjoy myself."