Is a mother to be obeyed in her command not to visit one's maternal uncle, and is it permissible to lie to her about visiting him, given the son's awareness of her injustice in this animosity and that the uncle does not pray?

Obedience to one's mother and kindness to her are obligatory, but her obedience is restricted to what is good and lawful. If a mother commands her child to cut ties with a relative (qat' ar-rahim), he should not obey her; for maintaining kinship ties (silat ar-rahim) is obligatory. Visiting a disobedient relative is permissible, unless shunning him would lead him to pray. Abandoning prayer is a grave sin, so your maternal uncle should be advised and encouraged to pray. It is not permissible to lie to your mother about this matter, but one may use ambiguous statements (ma'areedh).