Would the woman who is providing for her mother be sinful if she stopped giving her a monthly amount, especially since she has become engaged and her expenses have increased, and her father works and does not give her any money?

Righteousness towards parents is a religious obligation, and it is a path to Allah's pleasure. You must please your mother, treat her kindly, and obey her in what is good. It is not obligatory for you to spend on her as long as she is not in need, and there is no sin upon you for not giving her this amount, especially if you are in need of it and she is not. Nevertheless, it is advisable to seek reward in giving her money and not to discontinue this kindness towards her.