"My brother can't wait weeks or months. He is dying now," his sister Iman Kassem said in a statement this week.
Kassem started his hunger strike in September when he was sentenced to 15 years in jail in a mass trial with more than 700 co-defendants: "I am losing my will and don't know how else to get your attention," he wrote in those letters to Trump and Pence, adding that while he knows "full well that I may not survive," he had no choice.
Kassem is fed a liquid-only diet, but his health has greatly deteriorated, especially given his diabetes.
"Given his fragile health, we are very concerned," said Praveen Madhiraju, Kassem's lawyer and executive director of Pretrial Rights International, who added that Kassem has lost significant weight, been losing his hair and several times lost consciousness.
On Sunday, Madhiraju said Kassem's family was denied a weekly visit for the second week in a row.
"This is very worrying," Madhiraju wrote ABC News. "We haven't had independent verification of his condition for 2 weeks now."
They are urging the Trump administration to do more to pressure Sisi, who the president is close with, and secure Kassem's release.
A dual U.S. and Egyptian citizen, Kassem was visiting his wife and two children, then 3 and 6 years old, in Aug. 2013. It was a particularly volatile moment in Egypt's recent history -- one month after the military seized power following days of protests against the recently elected government of Mohamed Morsi.
In Morsi's place, then-General Sisi took control, implementing a crackdown on political opposition and civil society that has since expanded. About 20 Americans currently are in Egyptian jails, but there are as many as 60,000 political prisoners across Egypt, according to a Human Rights Watch report in 2017.
Sisi denied there were any political prisoners in a recent interview with CBS News.
On Aug. 14, 2013, the night before Kassem was set to return to the U.S., he went out in Cairo to exchange some money and shop, when security officials detained him and accused him of participating in protests against the military takeover in a nearby square, according to Madhiraju. The military was cracking down on the demonstrations in what human rights groups say was the single deadliest incident in Sisi's sweep to power, with as many as 800 killed.
Accused of being an American spy because of his U.S. passport and beaten by security forces, Kassem has been imprisoned ever since. His lawyers have called all the charges against him bogus.
"His imprisonment is only one example of Egypt's out-of-control security state, which has imprisoned tens of thousands for expressing political opinions or even for just being in the wrong place at the wrong time," said Stephen McInerney, executive director of the Project Project on Middle East Democracy, a nonpartisan organization that advocates for democracy in the region.
Just this week, the Egyptian parliament voted overwhelmingly in favor of a constitutional amendment that would allow Sisi to remain in power until 2034. After winning reelection in April 2018 in a vote with no real opposition and where his biggest opponent was jailed, Sisi's second term is currently set to expire in 2022, when he would be forced to step down.
In recent months, Kassem and his lawyers have given up appealing his case and filed paperwork to renounce his Egyptian citizenship, so that he could be deported. But at least three times now, according to Madhiraju, Egyptian authorities have denied that he has submitted paperwork, prolonging his detention.
The Egyptian embassy in Washington did not respond to request for comment on Friday.
Kassem's case was raised publicly and directly with Sisi by Pence when he visited Cairo in January 2018: "President Al Sisi assured me that he would give that very serious attention. ... I told him we'd like to see those American citizens restored to their families and restored to our country," he said at the time.
Kassem's sister Iman pleaded with Pence directly, "For him, his wife and his children, I'm asking Vice President Pence to please bring him home."
But there are concerns that Secretary of State Pompeo did not raise his case when he met Sisi more recently in January. Madhiraju told ABC News that he was told Pompeo did not, a claim that ABC cannot independently verify.
When asked, State Department deputy spokesperson Robert Palladino said in a statement, "We are deeply concerned by the conviction and sentencing of U.S. citizen Moustafa Kassem and have raised his case repeatedly with the Egyptian government both here and in Egypt."
Palladino said the U.S. is "concerned about the toll" on Kassem's health and remains "in communication with Mr. Kassem, his family and his attorney about the case and will continue providing appropriate consular service."
Consular officials from the U.S. embassy have been able to visit Kassem on multiple occasions over the last few years, Madhiraju said.
But on whether Pompeo specifically raised the case, a State Department official would only add, "While we don't discuss private diplomatic conversations, Mr. Kassem's case has been raised at the highest levels."
During his time in Cairo, Pompeo was asked about raising detained Americans' cases and told reporters, "We talk about the full panoply of human rights issues each time we engage."
"His case shows the willingness of President Sisi's regime to ignore legitimate concerns expressed by the Trump White House for an American citizen wrongly imprisoned in awful conditions and in dangerously poor health," said McInerney, adding that the fact that Pompeo may not have raised Kassem's case "gives the appearance that this administration has lost interest in the fate of an American citizen in critical condition."
Critics have said the Trump administration has been quieter on human rights and cozier with strongmen, from Vladimir Putin of Russia to Recep Tayyip Erdogan of Turkey to Sisi in Egypt.
After withholding $195 million in aid to Egypt over human rights concerns in 2017, the State Department announced in July it was releasing that money "in the spirit of our efforts to further strengthen this partnership," an official told ABC News at the time. The same week Kassem was sentenced, the administration announced it had approved the possible sale of $99 million worth of tank rounds to Egypt, calling it a "friendly country" and "important strategic partner."
Annually, the U.S. typically provides Egypt with more than a billion dollars in aid and military assistance -- the second highest amount behind Israel.
But Trump's tight bond and warm words with Sisi have yielded some results, too.
In April 2017, Egypt freed Aya Hijazi, a U.S. citizen and humanitarian aid worker, her husband Mohamed Hassanein and four others after Trump and his top aides urged Sisi to do so as a goodwill gesture.
For Kassem, there still hasn't been an Oval Office celebration.
He hasn't celebrated a birthday in years, either. On Monday, he turned 54 years old, and when his family tried to visit him to mark the occasion -- his sixth birthday behind bars -- they were forced to wait nine hours and then ultimately blocked by Egyptian officials, Madhiraju said.
Editor's note: Because of a calculation error, the number of birthdays Moustafa Kassem has had while detained in Egypt was incorrect. He has had six birthdays behind bars.