Thaksin Shinawatra said (yes, through Skype again to Pheu Thai MPs) there was no such thing as a "stand-in prime minister." Yes, her other sister, Yaowapa, is running in Chiang Mai's by-election but that's because she will be active in Parliament to help Pheu Thai Party's MPs get into line. She isn't being groomed as the next premier if a political accident materializes that could force his other sister, Premier Yingluck, out of office.
Yingluck herself told reporters that she wanted to complete her four-year term and that she wanted to work for the people. Regarding the Counter-Corruption Commission's investigation into her loans amoungint to Bt30 million to her husband Anusorn Amornchat's company, Ad Index, the PM said she had submitted all relevent information and was confident that she hadn't done anything wrong.
Thaksin's son, Panthongtae, however, wrote in his Facebook that Pheu Thai Party could come under pressure from various sources of ill intention. Uncertainty is always high. "So, it might be nice to have two or three stand-in premiers, just in case..."
You would have to decide for yourself which version is more credible. I am at my wit's end.