Does removing the upper part of a grandmother's elevated, marble-built grave constitute an insult to the deceased, and was that a mistake warranting cursing from relatives?

The Sunnah is to raise the grave about a handspan above the ground, making it humped, as reported from the Prophet (peace be upon him) and his Companions.

The hadith, "and no elevated grave but that you level it," means leveling it with other graves to about a handspan, or making it good according to the Sharia.

Building domes and shrines over graves, and raising them more than a handspan, is forbidden because it leads to their glorification and associating partners with Allah through them, and the ignorant believing in their ability to bring benefit and avert harm, just as the people of Jahiliyyah used to do with idols.

Therefore, demolishing marble and removing the upper part of the grave is good and consistent with the Sunnah. It is not considered disrespect to the grave or its occupant; rather, it honors them by following the Sunnah. One must be patient with any harm that may befall them when enjoining good and forbidding evil, while ensuring that it does not lead to a greater mischief.