Is it sinful to sit on a grave?

Just as a living person is respected, so too is the deceased. Therefore, the dead should be shown respect and one should avoid behaviors that imply disrespect. In this regard, unless there is a necessity, it is not religiously appropriate to walk over graves or sit on them. The Prophet Muhammad (peace be upon him) said, “For one of you to sit on a burning coal that burns his clothes is better than sitting on a grave.” (Muslim, Janaiz, 96 [971]). However, there is no objection to sitting beside the grave.

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu