Is it considered unlawful to deposit money in a bank and receive a fixed monthly interest from it, then spend it on the family? And what is the legitimate alternative if this interest is unlawful, given that there is no experience in trade and no fixed income?

No soul dies until it has completed its provision (rizq). A Muslim must be graceful in seeking their provision by pursuing lawful means and avoiding forbidden things like usury (riba) and gambling, as these do not increase provision. Allah has threatened those who consume usury with war and the wiping out of blessings. Allah has promised the pious a way out and provision from where they do not expect it. There are lawful alternatives for investing money, such as Islamic banks that adhere to Sharia principles, or entrusting money to a trustworthy and skilled person to invest it in exchange for a share of its profits.