The shootings occurred on Sunday at 6pm on the man’s farm in the rural village of Luthern, west of the city of Lucerne, cantonal police said.
The man opened fire on the woman and his brother in a shed, where they were milking cows, before pulling the trigger on himself, according to the preliminary evidence.
The woman was fatally wounded while the brother was injured in the arm, police said.
He ran to neighbours who contacted police.
Officers arrived at the farm to discover the gunman seriously wounded.
He was transported to hospital where he died the next day, police said.
Investigators are still trying piece together the reasons for the attack.
According to a police spokesman, the woman was a native of the region who occasionally helped out on the farm, the SDA news service said.
The Blick newspaper reported online that the gunman lived alone with his mother on the farm after his father, a local butcher, died of a heart attack.
The younger brother did not live on the farm but looked after the animals, the newspaper said.
A neighbour described a “tense” relationship between the two brothers after their father died, Blick reported.