The event in Wiltshire was the qualifying meet for the South West regional junior triathlon teams.
The top four male and female athletes from the region at Bowood are due to be selected to compete at the IRC event, which will take place as part of the British Under-20 Triathlon Festival at Leicestershire’s Mallory Park in September.
All 10 South West counties had athletes competing in Wiltshire at weekend, alongside other athletes from around the country and the Channel Islands.
Thomas, 13, was first boy home from the South West in the tri-star 3 category and came fourth overall. The City of Plymouth athlete and Dinnaton Swimming Club member finished in a time of 35.50 minutes, with a 6:07 minute 400m open water swim, a 17.35 minute 9km bike ride and an impressive 8.21 minute 2.5km run.
Sister Madeleine, 11, was one of the youngest competitors in the tri-star 2 category, but finished second South West athlete. She came home in an overall time of 30.30 minutes, with a 4.54 minute 300m swim, a 13.42 minute 6km bike ride and a 7.45 minute 1.9km run. Like her older brother, Madeleine also runs for City of Plymouth and swims for Dinnaton.
The duo had already impressed at the Bath Duathlon and Vale Tridents Triathlon and Duathlon competitions this year.
Before the Inter-Regional Championships, the South West will hold a three-day training camp at Mount Kelly College.
Meanwhile, Mount Kelly triathlete Tom Brew won the Henley Classic open water swim at the weekend.
Brew won the 2.1 km endurance upstream River Thames swimming event by 15 seconds, clocking 27.40 minutes.