Chart Thai Pattana director Nikorn Chamnong said on Saturday that Paradorn and Korawee Prissananthakul submitted their resignation on Friday.
Nikorn said he had no idea which party the two would join.
Their father, who is a veteran politician, resigned from the party earlier this week.
A source from the Bhum Jai Thai Party claimed that the two brothers would submit applications to become Bhum Jai Thai members at 10am on Monday.
The source said Bhum Jai Thai leader Anuthin Charnweerakul would be present to welcome the two brothers and accept their applications himself.