Is it considered a sin for the questioner to go have dinner at his uncle's house after his grandmother's death and before her burial, at a table that includes relatives of the deceased?

If the paternal uncle prepares a usual meal and does not intend it for the occasion of the deceased's passing, there is no harm in eating from it. As for what was reported from Jarir ibn Abdullah al-Bajali (may Allah be pleased with him) regarding considering gathering at the deceased's family and preparing food as a form of wailing (niyaḥah), what is meant by it is the deceased's family preparing food for people, and it is disliked (makrooh) in a way that is closer to discouragement than prohibition. It is sunnah (definition) to prepare food for the deceased's family and send it to them, not for those who gather with them. Their doing so for people is disliked, unless there is a need, such as guests arriving from distant places who cannot be fed otherwise.