ELKE Vermeulen has just started PhD work about wallabies and her sister Anke represented Australia in the 2012 World Broomball Championships in Canada.
The Belgian-born sisters from Kellyville took their Australian citizenship pledges on Thursday.
Same for their brother Stijn and parents Hermana and Jozef.
They joined more than 160 Hills residents from 29 countries in becoming Australian citizens at a ceremony at Castle Grand, Castle Hill.
"We moved here as a family so, for us, we wanted to do this as a family," Hermana said.
"We've been planning this since we arrived [on January 3, 2007]."
Jozef said he believed it would make them "part of the Australian family" and for that reason was "the right thing to do".
Did you become an Australian citizen in the lead up to Australia Day? What prompted your decision? Tell us below.
Said Hermana: "My favourite thing is summer but my husband, he says the clean air."
The automotive engineer rides his motorcycle to work at Castle Hill.
"For Anke a whole new world opened up for her by coming to this country," Hermana said.
"She has said many times that she would never have gone to university before coming here."
Anke, a former Wyndham College student, has completed forensic science and is waiting to join the police academy.
Her twin Stijn is studying design and animation at Grantham College.
"I'm going back to study. That's the only downside of coming to Australia, because my degrees [in physiotherapy] are not recognised," Hermana said.
Thursday's citizenship ceremony will be preceded by an invitation-only dinner where the The Hills' Citizen of the Year awards will be announced.
Hornsby Council holds its citizenship ceremony on Australia Day at Hornsby RSL, where 201 people, including 40 children, will receive their citizenship.
New citizens of The Hills by country of birth:
South Korea - 25
United Kingdom - 25
China - 16
South Africa - 15
Philippines - 10
India - 11
Iran - 9
Sri Lanka - 7
Belgium - 5
Netherlands - 5
Brazil - 5
Singapore - 5
New Zealand - 4
Fiji - 3
Hong Kong - 3
Malaysia - 2
Lebanon - 2
Thailand - 2
Bangladesh - 1
Czech Republic - 1
Burma - 1
Nepal - 1
Italy - 1
Ghana - 1
Colombia - 1
Moldova - 1
Bahrain - 1
US - 1