How long ago did he die?
She and the adult children should consult a solicitor to find out how to proceed to settle his affairs (maybe all go together so that costs can be shared?). She should also consult on her own to find out how their young children can be provided for (and what they stand to inherit), and where she stands legally.
His share of the family business should be split between his older children and the young children (no idea if she is entitled to a share) - and hopefully legal action will not have to be taken for that to happen.
She should also find out if his adult children can claim a portion of his share of their house.
I have no training, so really cannot say what, if any, legal rights she has. Also, we don't know what (if anything) they owned jointly.
Maybe someone with professional knowledge will see this and post more useful information.
In the meantime, I would think she should compile all business papers - bank statements, pension information, savings accounts, tax returns, mortgage papers, etc. Also look for legal documents regarding the business - perhaps there is something that indicates his role and what was intended?
It is a terrible situation to be grieving and to have an uncertain financial future.