You must know what he does for a living so you must have some rough idea of what he earns, surely?
Don't you ever sit down with him and disacuss your future plans i.e. when you will have another child, when you will move to a larger house, when and where you will go on holiday? If not, then he's sailing through this marriage without giving your views, hopes, aspeirations etc any consideration whatsoever.
That's not a marriage. That's a lucky bloke who is free to do what he likes with his time and money while the unpaid skivvy looks after his offspring.
Re the will. If he made it before marriage it's invalid. If he doesn't make adequate provision for you and your children, then you can challenge it, but you'll erode the value of the estate by doing so. If he doesn't have a valid will that is a very precarious position for you to be in. Ideally, as a married couple you would have 'mirror wills' that provide for either of you and the children in the event of the other's death.
Why he is intent on leaving some of the estate to his brothers rather than all to his wife and children. Is this a cultural norm of some description?
Personally, I don't think you have a marriage worth saving when communication is so poor and his views appear to be so entrenched.