Does a grandmother incur sin for prioritizing her son over her mother in food, thinking that she ought to have presented her mother before her son?

There is no blame on the grandmother for preferring her son with food over her mother, due to his greater weakness and need. However, she is sinful for her lie, and what is obligatory upon her is truthfulness. We hope that Allah will forgive her due to her regret and sorrow, for regret is repentance. She must seek forgiveness and frequently supplicate for her mother, as this is a form of righteousness towards her after her death.