Is the aforementioned behavior—including not disclosing the amount given by the mother—considered an act of disobedience to parents (ʿuqūq), or does it fall under the purview of the sayings of the Messenger of Allah, peace and blessings be upon him, and the verses of the Quran?

You have done something impermissible by denying that you took money from your mother and by telling your siblings that you spend on her from your own money, due to the lies involved, your disobedience to your mother’s command, your claiming what you were not given, harming your mother, and making her feel indebted to you. This could be considered an act of disobedience (ʿUqūq) if your mother was severely distressed, but we hope you will not be held accountable for it as long as you did not intend it. The door of repentance is open, so strive in repentance and make up for your past shortcomings in honoring your mother by supplicating for her, seeking forgiveness for her, giving charity on her behalf, paying off her debts, maintaining ties with her relatives, and honoring her friends.