Shamsun Nahar and her son Sajjadul Karim Shaon were found murdered on Wednesday in their at the capital's Kakrail.
Prime suspect Al Amin Jony has been arrested early on Saturday from Gopalganj's Maksudpur, said RAB-3 chief Lt Col Md Emranul Hasan.
Following the murders, police detained Shamsun Nahar's husband businessman Abdul Karim.
The next day police arrested a woman, Sharmin Aktar Mukta, and said she is the third wife of Karim, whose business concerns range from grocery to producing films.
Prime suspect Al Amin is Mukta's brother.
"He has confessed to the murders and revealing further information," said senior RAB officer Hasan adding details will be provided in media briefing later.
Police said Karim married her 28 years ago. Later, he married a woman named Farida, but the marriage did not last. Four years ago, Karim married Mukta.
The murder case started by Shamsun Nahar's brother Ashraf Ali accuses Karim, Mukta, Al Amin and several other unidentified people.
According to the case details, Mukta had come to Shamsun Nahar a few months ago and threatened her.
Quoting witnesses, plaintiff Ahsraf Ali claimed that Al Amin Jony along with few others murdered the mother and son.