Natural Resources and Environment Minister Varawut Silpa-archa has been elected leader of the 12-MP Chartthaipattana Party, replacing his elder sister Kanchana.
He also insisted that the party is not run by a political dynasty.
On Monday, Chartthaipattana convened a meeting of its executive board to find a replacement for Ms Kanchana, who stepped down as party leader on Sept 20.
During the meeting, Mr Varawut was elected party leader uncontested. The meeting also voted in 22 new executives, including Prapat Phothasuthon as the party secretary-general.
In addition, the executives also decided to lift the cap on the number of executives serving on the board from 29 at present to 39.
The board's expansion would prepare the way for the party to grow in the future, according to Mr Varawut.
Chartthaipattana currently has 12 MPs representing provinces in the Central Plain region.
At the meeting on Monday, Ms Kanchana presented a party flag to her younger brother in a symbolic leadership handover ceremony.
The siblings are the children of previous party leader Banharn Silpa-archa, who led what was then called the Chart Thai Party to victory in the 1995 election to become the country's 21st prime minister.
Banharn died on April 23, 2016, at the age of 83.
After the meeting on Monday, Mr Varawut said he would look after the well-being of party members just like he would his own family. He said these members have kept the party going.
Mr Varawut then denied claims by critics that the party appears to be the personal property of the Silpa-archa family.
"One must look back at the party before our family took the helm," he said.
"Previous leaders were Pol Gen Praman Adireksan and Gen Chatichai Choonhavan. No one can and will control the party," he said.
Mr Varawut said Chartthaipattana under his leadership will ensure members do take part in the party's decision-making processes.
The party has set its sights on winning between 25 and 30 MP seats at the next polls. Mr Varawut said he would have the party limit its growth to allow for the effective management of its affairs.
When asked if or how Chartthaipattana will be impacted if the ruling-Palanag Pracharath Party nominates its leader Gen Prawit Wongsuwon as a prime ministerial candidate in the next election, Mr Varawut said the issue may be discussed during the party's executive meeting on Thursday.
He insisted it was too early to speculate on the party joining a coailition government after the next election.