The Bernoulli Family:
Jakob Bernoulli (1654 - 1705) taught himself the new Calculus of Newton and Leibniz, as did his brother Johann (1667 - 1748). Jakob invented polar coordinates and made significant contributions to the theory of probability. Johann first studied medicine (taking a doctor's degree at the University of Basel, Switzerland) with a thesis on muscle contraction. He soon became fascinated with Calculus, applying these new methods to a variety of problems in geometry, differential equations and mechanics. Both brothers became professors of mathematics at Basel (with Johann succeeding his older brother).

There was a bitter rivalry between them which was aggravated when they worked on the same problem. In 1690, Johann posed the Brachistochrone Problem as a challenge to the mathematicians of the world:

A wire is bent in a curve which joins two points.
A bead slides down the wire without friction.
What curve will give the minimum time of descent?

It is said that Newton, after a day of work at the Mint, learned of the problem and solved it the same afternoon. It was also solved by Jakob Bernoulli and Leibniz.

Johann's son, Daniel (1700 - 1782) also gave up medicine for mathematics and went on to become an outstanding mathematical physicist. In three generations the Bernoulli family produced eight mathematicians and scientists, the above three being the most outstanding.

Note: The Brachistochrone curve is the cycloid: if a circle rolls without slipping on a flat surface, a point on its circumference traces a cycloid.

P.S. Besides Jakob, Johann and Daniel there were:

John (Jakob's brother) who occupied the chair of math at Groningen.
He stole results from Jakob
... and expelled his son Daniel from his house for obtaining a prize from the French Academy which he expected to receive

Nicholas, the eldest son of John. He lived in St. Petersburg, where he was professor.

Daniel, the second son of John shares with Euler the unique distinction of having gained the French Academy prize ten times!

John Bernoulli the younger ... and his sons John and James ... and maybe others?