Edith Casas insists Victor Cingolani did not murder her sister, a model with whom he also had a relationship.
Cingolani was convicted of Johana Casas’ murder and is serving 13 years in prison in the town of Pico Truncado.
The couple have said they plan to wed on Friday at the prison in the southern province of Santa Cruz, to avoid media attention.
Cingolani is reported as telling Argentine media outlets that his relationship with Johana was “casual”, but that he was “in love” with Edith.
“We have lots of things in common and she isn’t jealous,” Argentine newspaper Clarin quoted him as saying.
“We always talk about Johana, about how she was,” he said.
Edith maintains that Cingolani was unjustly convicted, saying he is “a guy who would not hurt a fly”.
Their mother, Marcelina del Carmen Orellana, said it was clear to her that Edith was “psychologically ill”.