Is inward resentment towards a mother who behaves in this manner considered an act of disobedience (uquq)? And what is the way through which the mother can be helped to stop her actions and return to God and her husband?

Disliking one's mother due to her disgraceful actions is not considered an act of disobedience (aqūq), because matters of the heart are beyond human control. However, one must be cautious that this dislike does not lead to any mistreatment, as that would indeed constitute aqūq. Advising parents with gentleness, kindness, and wisdom is not aqūq. You have done well by patiently enduring her and advising her. We recommend that you pray for her repentance and guidance, and seek out people of virtue to advise her. We also advise you and your siblings to maintain strong ties of kinship amongst yourselves.