They continue to push Mexican authorities to charge Monica's husband, Bruce Beresford-Redman, with her murder.
"We are coming together in pain and we are trying to do the best that we can at the moment because we are grieving deeply," Monica's sister Jeane Ferreira-Burgos said Tuesday in an interview with "Good Morning America." "There's a lot of sorrow going on and a lot of pain, but we need to keep strong so we can bring justice to my sister."
Monica Beresford-Redman and her husband went to Mexico to work on their troubled marriage. Friends said they were on the verge of divorce after Monica discovered her husband was having an affair.
E-mails between Bruce Beresford-Redman, his wife and the alleged mistress have surfaced, highlighting details of a marriage in turmoil.
While staying at Moon Palace Golf and Spa Resort on night of April 5, witnesses said they heard the couple arguing. Three days later, on what would have been Monica's 42nd birthday, her body was found in a sewer near the resort.
Strangulation was confirmed as the cause of death. Authorities said she had also received a heavy blow to the head, and her husband had scratch marks on his arms and neck.
"In any jurisdiction it would be frustrating that it's been three weeks and there hasn't been an arrest. But we're hopeful that there's been progress made," said Alison Triessel, the family's attorney.
As a person of interest, Bruce Beresford-Redman was barred from leaving Mexico and was believed to be staying in the town of Merida, a four-hour drive from Cancun. So far he has not commented publicly about the murder.
"We're also happy to learn that he will not be given his passport back until all due diligence has been taken and a thorough investigation has been completed," said Triessel.
As for the couple's two children, Bruce's parents have filed for custody, but Monica's family argues that the children belong with them.
Although the Hollywood producer has not been charged in the murder, Monica's sisters are convinced she died at the hands of her husband. Monica's body was expected to be flown back to Los Angeles on Wednesday.