A Gold Coast mother killed two of her vulnerable adult children for financial gain, before trying to claim more than $100,000 in death benefits, police will allege.

Maree Mavis Crabtree, 51, allegedly forced prescription medication on her 18-year-old daughter and 26-year-old son, killing them in what initially appeared to be suicides.

Maree Mavis Crabtree has been charged with murdering two of her children.

Ms Crabtree is originally from Sydney and the family was believed to have only moved to Queensland in 2011, so police are appealing for witnesses from all around Australia to come forward with any relevant information they may have about the family.

A friend tasked with looking after the family's Gold Coast home, while the rest of the family was away on a cruise, found Erin Crabtree’s body on September 4, 2012, sparking an investigation police said was never closed.