The sister's boss was among those taken into custody, and a candidate for the local council in the northeastern city of Campina Grande was being sought in connection with the crime, Brazilian police said on Wednesday.
Angelica Aparecida Vieira was released unhurt on Tuesday, a day after being abducted in front of the restaurant where she worked as a nutritionist.
Police said the 22-year-old Vieira was released after some of the suspected kidnappers got into a disagreement about the ransom amount. One of them was upset and decided to let her go.
Police inspector Marcos Paulo said the group was going to ask for a ransom of about $150,000, but problems began when they decided the money would not be divided equally among those involved.
"One of the suspects didn't like the amount he was going to receive and decided to let her go," Paulo said. "Angelica actually helped convince him that it wasn't enough for his troubles and even said that she could give him more money if he let her go."
Authorities said the kidnapping was apparently orchestrated by Vieira's boss and the council candidate because they had debts to pay. The candidate's car was allegedly used in the kidnapping.
"They thought that Angelica's kidnapping would lead them to easy money," Paulo said.
It was the boss who actually first contacted police about the kidnapping, saying that she disappeared after he left her in front of his restaurant. But police started to doubt his story and the investigation eventually led to him.
"Everybody talked and they all confessed," Paulo said.
Hulk, who plays for Zenit St. Petersburg, had been advised by police to stay in Russia and was not expected to return to Brazil after news his sister was safe.
The 26-year-old striker moved from FC Porto to Zenit in September in a deal reportedly worth €55 million, becoming one of the highest ever transfers for a Brazilian player. He has been constantly selected for Brazil.
Relatives of famous Brazilian players are frequent targets of kidnappers.
In 1994, Romario's father was abducted and spent seven days in captivity before being released, and the mothers of Robinho, Luis Fabiano and Grafite have been abducted within the last 10 years, as was the sister of former Real Betis forward Ricardo Oliveira. None of them were hurt.
Follow Tales Azzoni at http://twitter.com/tazzoni