"Alaa deteriorated severely in the past two weeks, but at least they got to see him, and he needed to see the family so much," his sister Mona Seif wrote on Twitter.
She said news from the visit was "unsettling."
The family has not disclosed more information about the visit or Abdel Fattah's status, but "will share the full details later," Seif added.
Thursday was the first time the activist's mother Laila Soueif was allowed to visit him in nearly a month, after prison authorities repeatedly denied her access last week.
The pro-democracy blogger is currently serving a five-year sentence for "spreading false news" by sharing another user's Facebook post about police brutality.
After seven months consuming what his family said was "100 calories a day," Abdel Fattah escalated his strike to all food, and then water on November 6 to coincide with the start of the UN COP27 climate summit in Egypt.
In a letter handed to his family on Tuesday, but dated Monday, he said he had ended the strike.
"I want to celebrate my birthday with you," wrote the activist, who turns 41 on Friday.
Egypt's turn hosting the UN climate summit is coming to an end this week, with Abdel Fattah continuing to make global headlines as an example of what rights groups call Egypt's "abysmal" human rights record.
World leaders have raised his case in bilateral meetings with President Abdel Fattah al-Sisi.
The dissident's aunt, novelist Ahdaf Soueif, wrote on Twitter on Tuesday "Alaa had no idea the size of the support surrounding him."
Rights groups estimate Cairo is holding about 60,000 political prisoners, many of them in brutal conditions and overcrowded cells.