German philosopher, economist, historian, sociologist and political theorist Karl Marx (1818-1883) did not have a sister called “Onya Marx” (“on your marks,” as said at the start of a race), but there are jokes.
     
”@amazinghaynes Did you know Karl Marx had a sister called Onya?” was posted on Twitter by lauren hogan on April 29, 2010. “@RealBobMortimer True. Just like Karl Marx’s long lost sister, Onya” was posted on Twitter by lauren hogan on June 7, 2011. “@RealBobMortimer Almost as good as ‘True or false : Karl Marx has a sister called Onya?’” was posted on Twitter by David Taylor on November 25, 2011. The joke was probably started by English comedian Bob Mortimer. The comedy team of Vic and Bob (or Reevers and Mortimer) also told the joke in 2018.
     
“The starting pistol was invented by Karl Marx’s sister Onya” was posted on Twitter by lauren hogan on October 9, 2017. “Karl Marx is a historically famous philosopher but no one ever mentions his sister, Onya, the inventor of the starting pistol…” was posted on Twitter by The Dad Joke Man on July 21, 2020. “Never realised until today, that Karl Marx’s sister Onya, invented the starting pistol….” was posted on Reddit—Oneliners on July 24, 2020.
 
“While Alan Turing was cracking codes in WW2, his sister Kay was providing drinks & sandwiches” (Kay Turing = catering) is a similar type of joke.
       
           
Wikipedia: Starting pistol
A starting pistol or starter pistol is a blank handgun that is fired to start track and field races, as well as competitive swimming races at some meets.
 
Wikipedia: Vic and Bob   
Vic and Bob, also known as Reeves and Mortimer, are a British comedy double act consisting of Vic Reeves (born 24 January 1959, real name James Roderick Moir) and Bob Mortimer (born 23 May 1959). They have written and starred in several comedy programmes on British television since 1990, with Reeves having made his first TV appearance in 1986.
 
Twitter
lauren hogan
@_LaurenH_
@amazinghaynes Did you know Karl Marx had a sister called Onya?
8:00 AM · Apr 29, 2010·Twitter Web Client
   
Twitter
lauren hogan
@_LaurenH_
Replying to @RealBobMortimer
@RealBobMortimer True. Just like Karl Marx’s long lost sister, Onya.
1:19 PM · Jun 7, 2011·Twitter for iPhone
   
Twitter
David Taylor
@DAVID41184
Replying to @RealBobMortimer
@RealBobMortimer Almost as good as ‘True or false : Karl Marx has a sister called Onya?’ 😊
10:21 AM · Nov 25, 2011·Twitter Web Client
   
Twitter
lauren hogan
@_LaurenH_
The starting pistol was invented by Karl Marx’s sister Onya.
7:17 PM · Oct 9, 2017·Twitter for iPhone
     
Twitter
Vic and Bob
@IsItTrue_false
True or false?
Karl Marx had a sister called Onya ?
#vicandbob #reevesandmortimer #shootingstars
9:31 AM · Jul 8, 2018·Twitter Web Client
 
Twitter
Gary Lee Wadsworth
@Donny_Jpeg
Just laughed out loud on London Bridge at the @RealBobMortimer s fact that Karl Marx had a brother called Onya.
2:59 PM · Aug 10, 2018·Twitter for Android
   
Twitter
Dan
@SpursManDan
Karl Marx had a sister called Onya.
Brilliant from Bob Mortimer.
4:11 PM · Aug 23, 2018·Twitter for iPhone
 
Twitter
The Dad Joke Man
@DadJokeMan
Karl Marx is a historically famous philosopher but no one ever mentions his sister, Onya, the inventor of the starting pistol…
1:57 PM · Jul 21, 2020·Twitter for iPhone
     
Reddit—Jokes
Posted by u/Brucemoose1 July 21, 2020
Karl Marx is a historically famous philosopher,
however no one ever mentions his sister, Onya, the inventor of the starting pistol…
 
Reddit—Dadjokes
Posted by u/Gattica97 July 23, 2020
Karl Marx is a historically famous philosopher, but no one mentions his sister
Onya, the inventor of the starting pistol
 
Reddit—Oneliners
Posted by u/VERBERD July 24, 2020
Never realised until today, that Karl Marx’s sister Onya, invented the starting pistol….