Sian (18) won the women's under-20 title, putting the hammer down over the final stages after a battle with Auckland champion Ashleigh Williams to clinch victory in 17min 52sec, with Williams just 4sec behind.
The combined field featuring the masters women's grades also contained the women's under-18 grade, in which Hanna English was competing.
Hanna (16) found herself in a one-on-one battle for the under-18 crown with another Aucklander, Alana Sherman.
Sherman gradually pulled away over the closing stages to clinch the title in 18min 7sec. Hanna was forced to settle for the silver medal in 18min 13sec.
The sisters are coached by Mike Weddell, and Saturday's race was the first time this winter Sian has had the better of her younger sister after following her home in the Edmond Cup and Otago cross-country competitions.
Matt Prest (Canterbury) won the junior men's under-18 grade over 6000m in 19min 26sec. The victory was the Barry Magee-coached Prest's first regional or national title.
Other junior races were also dominated by Canterbury runners: Ben Musson (men's under-20), Ari Graham (girls under-14), sister Mya Graham (girls under-12), Chris Dryden (boys under-14) and Oscar Fossey (boys under-12).
Meanwhile, Louisa Andrew's golden run continued with an impressive victory in the women's 35-39 age-group.
The Otago runner's time of 17min 56sec was not only a personal-best over 5000m, but was only bettered by Sally Gibbs (Waikato Bay of Plenty), who ran 17min 17sec to win the combined masters grades section and the women's 50-54 age-group.
Andrew (39) has had the Midas touch this year, winning everything she has entered.
''It was fast from the word go,'' Andrew said of Saturday's title race, in which Gibbs and Gabrielle O'Rourke (Wellington) made a flying start.
Andrew's goal was to stay as close as she could to O'Rourke and see what happened. She also soon became aware of the presence of fellow Otago representative Shireen Crumpton.
With 2000m left to go Andrew thought she was a spent force.
''I didn't think I had any more. But somehow I just had a bit of a kick at the end and passed them both. I couldn't believe it.''
O'Rourke finished third overall and claimed the women's 45-49 age-group title in 17min 58sec. Crumpton, who was fourth overall in 18min 4sec, claimed the women's 40-44 title.
Richard Bennett (Canterbury) overcame a strong challenge from Brian Garmonaway (Wellington) to win the open masters grade and claim the 45-49 title.
''I tried to just run honest and keep it a nice pace,'' Bennett said at the finish.
Bennett and Garmonaway saw off strong challenges over the first half of the 10,000m race, with Bennett just breaking away from Garmonaway in the shadows of the finish line to record 32min 49sec. Garmonaway was second in 32min 54sec and Simon Mardon (Nelson) third in 32min 59sec.
