Is it permissible for me to financially assist my brother with his marriage, knowing that my mother believes this might lessen his sense of responsibility towards his new family?

What the mother sees as right is that a person should be independent and rely on themselves, not coveting what others possess. Therefore, if helping the brother increases his laziness and encourages dependence, he should not be helped until he relies on himself and earns his livelihood through his own effort. However, if he has obligations or is in dire need, then it is advisable to help him, and it is recommended to take precautions in the future so that he does not become a burden on others.