Abdel Fattah's letter, dated November 12, says he is "doing well, under medical supervision, and has begun drinking water," Ali wrote on Facebook.
The activist's sister meanwhile confirmed in a statement that "it is his handwriting", but asked "why did they hold this letter back from us for two days? Is it just cruelty to punish the family for speaking up?"
Echoing demands she made during COP27, Seif said her brother "needs to be on a plane to London and only then will we allow ourselves to feel true relief".
"Alaa is still on hunger strike, the UK embassy has still failed to achieve consular access, he's still arbitrarily detained with no end in sight."
Abdel Fattah, who consumed "only 100 calories a day" for seven months, escalated his strike, first to all food, then water as the COP27 climate summit opened on November 6 in the Egyptian resort town of Sharm el-Sheikh.
Abdel Fattah, a key figure in the 2011 uprising that toppled longtime dictator Hosni Mubarak, is serving a five-year prison sentence for "spreading false news" by sharing a Facebook post about police brutality.
He has been leading headlines since the UN climate talks began last week in Egypt, which sought to burnish its image by hosting COP27 but has come under fire over its human rights record.
Attending the summit to campaign for her brother's release, Seif was last week heckled by pro-government attendees, who called her brother a "criminal", not a "political prisoner".
A German diplomatic source said a complaint was lodged with Egypt as their delegation - which hosted Seif and other human rights defenders - "felt we were being watched."
International pressure has mounted since world leaders began arriving in Sharm el-Sheikh last week. Several raised Abdel Fattah's case in bilateral meetings with President Abdel Fattah al-Sisi, most recently US President Joe Biden on Friday.
Biden welcomed Sisi's reactivation of a dormant presidential pardon committee, which has facilitated the release of several high-profile political prisoners this year.
On Friday, Abdel Fattah's other sister Mona Seif announced the family had submitted a new request for presidential pardon.
The plea was picked up by one of Egypt's most watched talk show hosts, the ardently pro-Sisi Amr Adib.
On prime time television Friday, Adib said the pardon would be in "the interest of Egypt first and foremost."