The 53-year-old died last week after contracting the Delta variant of coronavirus.
He had been placed in an induced coma in a Manchester hospital earlier in the month as the group were forced to cancel their UK Christmas tour.
However, member’s of Marin’s family have now said that they are “convinced” he would have lived if he’d been in his home country of Spain.
Lawyer Alberto Martin told local Spanish media: “I spoke with his mum the day he died and she was saying she was convinced that if had been in Spain, he could have survived.”
He continued: “It’s something that we’ll have to analyse when we see all the medical documents because we feel there was time to control his situation and improve things so he could pull through and have his life saved which was the most important thing… The family are not happy with anything because they’re heartbroken.”
A public wake will be held for Marin in Madrid on Tuesday (28 December).
The operatic pop group Il Divo were put together by Simon Cowell in 2003, with members originating from Spain, Switzerland, France and the US.
Over the years, the group released 10 studio albums, which have sold more than 30 million copies worldwide.
Announcing Marin’s death on Sunday 19 December, the band wrote on social media: “It is with heavy hearts that we are letting you know that our friend and partner, Carlos Marín, has passed away.
“He will be missed by his friends, family and fans. There wiIl never be another voice or spirit like Carlos.”