Jane, a Catholic, was cremated at her request. She was always a firm believer in the Resurrection and in Everlasting Life. Her son Jim has kept her cremains in a beautiful wooden container at his home in his mother’s former bedroom. Jim prays for his mother daily, and has Masses offered for her. Jim has specified in his will that Jane’s cremains are to be interred in an appropriate site upon his death. Is Jim committing a mortal sin by delaying the interment of his mother’s cremains?
From the USCCB:
The cremated remains of a body should be treated with the same respect given to the human body from which they come. This includes the use of a worthy vessel to contain the ashes, the manner in which they are carried, and the care and attention to appropriate placement and transport, and the final disposition. The cremated remains should be buried in a grave or entombed in a mausoleum or columbarium. The practice of scattering cremated remains on the sea, from the air, or on the ground, or keeping cremated remains on the home of a relative or friend of the deceased are not the reverent disposition that the Church requires. (Order of Christian Funerals, 417)
I don’t think I’d ventured to say that he’s committing a mortal sin. It appears, based on your post, that he thinks he’s treating his mother’s remains in a respectful manner. However, the Church asks that we treat the cremation remains in the same manner we would treat the body. I doubt he’d keep his mother’s body in the house, so in this respect he needs a little education on the importance and significance of properly interring her remains. This should be done with great sympathy and empathy for the sorrow and grief he is experiencing.