What is the ruling on one who does not wish to help his sick mother, and is the statement about sharing accounts in the Hereafter considered disbelief (kufr), and what is the ruling on one who laughs at a statement of disbelief without intention?

It is obligatory upon all of the grandmother's children to be dutiful to her and to serve her when she is in need. If your maternal uncles and aunts refrain from serving her without a valid excuse, then they are sinful. Your mother's act of serving her mother is a great righteous deed. Your statement that they will share the reckoning on the Day of Judgment is correct, as each individual will be held accountable for their deeds. One who laughs upon hearing words of disbelief, without mocking the religion, does not commit an act of disbelief. Beware of obsessive whispers (waswasah).