(CNN) It's not always easy to get along with your siblings. But it's essential if your sister's a professional golfer -- and you're her caddy.
Brittany and Brooke Henderson are the game's own sister act. At the age of just 18, Brooke became the youngest winner of the Women's PGA Championship, defeating Lydia Ko in a playoff in June. Brittany, seven years her senior, was on bag duty.
The Canadian climbed to No. 2 in the world rankings after that success, but how much of it was due to their special partnership?
Although they rarely make the headlines -- unless they work for Tiger Woods -- caddies are a constant influence over a player's tactics, technique and mood during tournaments.
"To be a great caddy you need patience and perseverance," Brittany Henderson told CNN's Living Golf show at 2016's final women's major, the Evian Championship, where Brooke tied for ninth place.
"Definitely get along well with whoever you're working for -- I think that's huge. It's really a partnership.
"On top of that, being good with math -- mostly just addition and subtraction."
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Before every tournament, the player and their caddy will walk the course, getting all the distances and checking that everything in the yardage book is correct.
"You have to really scout out where you want be and also where you don't want to be," Henderson says.
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"Usually, it takes a couple of hours to walk around the course and do that. Then you lay out the game plan for the player and make sure they have all the info they need."
This year such diligence paid off for the sisters -- Brooke fired a hole-in-one on the way to winning the PGA Championship, and gifted the car on offer to her sister.
Off the course, they also spend plenty of time together.
"At the end of the day we usually have a small decompressing. Sort of relax a bit. We grab a bite to eat and talk about some things that we could improve or change for the next day," Brittany adds.
"Brooke will generally practice if there's something she needs to work on, and then we try to get bed early."
Mark Twain is often attributed with the statement that "golf is a good walk spoiled" -- whether or not he actually said it, caddies know what he meant.
Henderson estimates that she walks six miles a day during a tournament -- 42 miles each week. That's a lot, she acknowledges, regardless of whether you're carrying a 45-pound bag on your back.
But that's not the worst part.
"My least favorite part of the job would be if we ever disagree," Brittany admits. "It doesn't happen very often luckily, but inevitably it happens occasionally."
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And how is it resolved? "We have a rule that it always comes down to the player," she says. "I try to give her mostly factual information.
"She'll ask my opinion, and if she disagrees with my opinion she's going to go with her gut. It's what the player always should do."
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Caddies such as Steve Williams -- who was on Woods' bag for most of his major wins and helped Adam Scott to a Masters title -- may enjoy the limelight, but Henderson confesses she likes not having the pressure of performing on a big stage.
"My favorite part is being able to contribute so much, and then not actually have to execute the shot."