Sunday will mark just the second time a pair of sisters have faced off in the NAB AFLW competition.

Breann (Carlton) and Celine (Western Bulldogs) Moody played each other last season, while Sunday will see Sarah and Jess Allan take to the field when Adelaide and Greater Western Sydney play.

The Allans have only ever officially played on opposing sides in local basketball. This weekend, they may actually line up on each other, with Giants ruck Jess (20) regularly resting forward and Crows defender Sarah (22) usually taking the tallest attacker.

"We played against each other a bit in the practice game, so I wouldn't be surprised if it happened again. It's highly likely, I think," Sarah told womens.afl.

"She hasn't kicked a goal yet, so as much as I want her to kick her first goal, I don't want it to be against me. I hope for the best for her, but on game day, we'll do the best for our own teams."

The two, who are just 18 months apart, lived together when Jess played for Adelaide in 2018.

Jess then took 2019 off due to work commitments with the army, but as she is now stationed in Canberra, has pulled GWS' orange and charcoal this year.

"She's loved being back and given the opportunity to play footy while pursuing a career in the defence force, which is a dream for her," Sarah said.

"We're pretty close. We don't talk too much about our teams, but we still talk individually to each other, like we would any other season."

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The pair both wear the number 39 for a touching family reason.

"I originally took the number on because it was our dad Keith's number at Centrals in the SANFL. I took it on as a bit of a thank you for all the time and support he put into our footy," Sarah said.

"Jess wore 25 at the Crows, but then changing to the Giants she also had to change her number, so she went with the 39 as well."

Looking more broadly, the game is also highly crucial in the futures of both teams this season.

Coming into the round, the Giants are sitting in third position, four points clear of the Crows in fifth place. Lose this one, and it's almost impossible for Adelaide to defend its premiership.

"We know it's an important game heading into the last few rounds. For us, we're just trying to keep it simple and we know we have a job to do, which is just winning, from here on out," the Crow said.

"It's funny how I'm playing my sister in a game where the winner will still be in the finals chase and the loser will struggle to make it."

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So, will there be any banter between the softly spoken but highly competitive pair?

"I highly doubt it, we're not really the sledger types. In the practice game I tried to have a bit of a chat to her, but she wasn't having a bar of that," Sarah said.

"I think she'll go in with her game face on and we'll be trying to do our best on the day, and then after we'll be back to normal.

"She's staying with me for an extra night, so we'll be able to have some good quality time after the battle's over.

"But if she wins, she might have to find somewhere else to stay for the night."

The match against GWS will be broadcast live on Channel 7 Adelaide from 2.30pm and nationally on Fox Footy and the AFLW app.