Hello Anna,
To be able to respond to you, we need a bit more context: Where are you applying for your visa? Did you receive your admission in a private or public institution? How much are your tuition fees? What is the size of your brother’s accommodation? How many people are in the accommodation?
When you say « the embassy is asking me again for proof of sufficient resources amounting to 615 euros », how did you receive this request? Have you already applied for your visa and then been asked for this proof? And when you mention the postal bank, I assume you are not yet in France, how did you manage to have an account at the postal bank? Unless it is not the French postal bank?
So many clarifications!
Also read: Studying in France: What documents are needed for a student visa?