Riham Dawabsha suffered severe burns to 90% of her body in the July attack, blamed on Israeli settlers.
Ali, her 18-month-old son, died during the attack while her husband, Saad, succumbed to his injuries a week later.
The deaths provoked international condemnation. So far no suspects have been arrested.
The Dawabsha family's home was one of two houses in the village of Duma set on fire and daubed with slogans in Hebrew, including the word "revenge".
Another son remains in hospital.
Riham's body is expected to be returned from the Israeli hospital where she was being treated to the Palestinian authorities in the occupied West Bank.
Palestinians have accused Jewish settlers of carrying out the attack; Israel has not said whether it holds settlers responsible, and an investigation is ongoing.
Israeli Prime Minister Benjamin Netanyahu has called the attack an act of terrorism and has said every effort will be made to catch the culprits
After the attack, the Israeli government announced new measures to combat vigilante Jewish groups who attack Palestinians and their property, including administrative detention for extremists.
A number of detentions have been made but none in direct connection with the firebombing in Duma.
A Palestinian woman has died of injuries she sustained during an arson attack on her home in the West Bank that killed her husband and infant son.