BANGKOK—Since Thailand’s army ousted Yingluck Shinawatra’s government from office, the former leader has largely kept a low profile, visiting temples and rarely talking about prosecutors’ charge that she wasted billions of dollars subsidizing rice farmers.

But now Ms. Yingluck and her brother Thaksin Shinawatra, another ousted former Thai prime minister, are thrusting themselves back into public view to challenge the charge and the military’s deepening hold on power here.

“People in power thought I would be an obstacle or a source of conflict, so I kept quiet,” Ms. Yingluck told The Wall Street Journal here Wednesday. Her change of tack, she said, reflected a new effort to prove her innocence during her trial, which began last month. A conviction carries a penalty of up to 10 years in prison.

Her brother Mr. Thaksin, a billionaire businessman whose swagger transformed Thai politics before he was ousted in a 2006 coup, has meanwhile instructed supporters in a Lunar New Year message from abroad to organize themselves after a long hibernation and distributed calendars and lavish coffee table books eulogizing his premiership.

The siblings’ growing profile in this tropical Buddhist kingdom is adding pressure to the country’s military leaders. Junta chief Gen. Prayuth Chan-ocha recently swore at a local press briefing where reporters asked him if he was sticking to his plan to hold elections in 2017. Members of the country’s main political parties have criticized a new draft constitution that would enhance judicial powers to remove elected leaders. The country’s economy has been troubled, too, with exports contracting in 2014 and 2015 and foreign investment falling.

From afar, the U.S. is looking on with concern, and in response is scaling back annual military exercises with Thailand, the latest of which began Tuesday. U.S. Ambassador Glyn Davies told reporters at the launch that he hoped the relationship between the two countries would improve if Thailand returned to democracy after 2014’s coup.