Kulsoom was diagnosed with early-stage lymphoma last month and has been in London since, where she has undergone three surgeries.
Her daughter, Maryam Nawaz, who is in London, tweeted, "Ami (mom) was taken & admitted to the hospital again late last night".
"Doctors are trying to stabilise her condition. Request for prayers," she said.
Kulsoom recently won Lahore's NA-120 seat vacated by Nawaz Sharif after he was disqualified by the Supreme Court in the Panama Papers case.
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Quoting doctors, the Sharif family had said that Kulsoom's cancer was curable.
Sharif, who was also in London for his spouse's treatment, returned to Pakistan on Monday, putting to rest all speculation that he would skip the corruption cases against him.
All other members of the family, including daughter Maryam and sons Hussain and Hassan, are still in London.