Ms Yaowapa won a by-election in Chiang Mai's Constituency 3 on April 21 last year by a landslide, securing 67,101 votes against Democrat contender Kingkan Na Chiang Mai's 21,372.
Ms Yaowapa, the younger sister of Thaksin Shinawatra and older sister of current caretaker Prime Minister Yingluck Shinawatra, made her return to parliament after being banned from politics for five years.
She was among 111 executives of the disbanded Thai Rak Thai Party who were suspended from politics for electoral fraud in 2006.
The by-election followed the resignation of Pheu Thai's Kasem Nimmonrat, who cited health problems.
Political observers believed Pheu Thai was grooming Ms Yaowapa as a potential candidate for prime minister should Ms Yingluck be disqualified for any reason.
Mr Somchai said Ms Yaowapa is accused of violating election laws by arranging for voters to be transported to advance voting on April 12 at polling stations in San Kamphaeng and Doi Saket districts.
Mr Somchai said if Ms Yaowapa is found to have committed wrongdoing, she could be red-carded, banned from politics for 10 years and face criminal charges.
In this case, no yellow card will be issued because the House is now dissolved and there will be no by-election. Those issued yellow cards are still allowed to compete in by-elections but those given red cards cannot participate.
He said an EC inquiry had finished its probe and the commissioners are expected to announce their findings next week. Mr Somchai also said the EC will today ask the prime minister's secretary-general Suranand Vejjajiva to testify over a case in which Ms Yingluck is accused of using state resources to campaign for votes.
He said the EC has received complaints that Ms Yingluck used state resources to drum up votes for her party during her inspection tours of areas in the North and the Northeast following the House dissolution on Dec 9, which paved the way for the Feb 2 general election.