Serena and Venus Williams.
Vitali and Wladimir Klitschko.
Andy and Jamie Murray.
It's hard to imagine how it would feel to compete in the same sporting field as your sibling, let alone compete against them.
But, Alastair Brownlee just proved the competition is a secondary concern when your brother is in need of help.
This morning, Alastair helped his exhausted brother Jonny cross the finish line in a dramatic end to the Triathlon World Series in Mexico.
With only 700m left to go and in the lead, the youngest Brownlee brother began to zig-zag along the road, at the point of passing out, due to the humid conditions in Cozumel, Mexico.
Spotting his brother was in trouble, Alistair grabbed Jonny's arm and propped him up as they ran the final length of the course, before pushing him over the line and securing Jonny a second place while he came in at third.
Er, someone give the guy a medal.
Wait, he won a medal?
Well, give him another one.
A win would've secured Jonny a world title but the hot conditions seemed too much for the Rio 2016 Olympic silver medallist, who was later given medical treatment.
Hours later, the triathlete tweeted a picture of himself hooked up to an IV drip, thanking his brother for his selfless act.
Following the race, Alistair – who was the 2016 Olympic gold medallist in the triathlon – said his urge to grab his brother and run to the finish line together was 'a natural human reaction'.
'I'd have done the same thing for anyone in that position,' he said.
However, Alistair didn't hold back when it came to teasing his brother's lack of energy on the final stretch.
'I wish the flippin' idiot had paced it right and crossed the finish line first.
'He could have jogged that last two kilometres and won the race.
'You have to race the conditions. I was comfortable in third. I raced the conditions, I took the water on, made myself cool and I was all right,' he added.
Guess the drinks are on you tonight, Jonny.
Watch the dramatic finish below: