French inheritance law is pretty strict and rigid. In the case you describe, the house will have to be shared equally among the 5 heirs (called an indivisible interest) - and in order to sell the property, all 5 will have to be in agreement.
How your brother is represented may or may not come into question, as long as he has some form of legal representation (like a power of attorney or a guardianship) that is recognized in the UK. If you have the power to act for him in the UK, I suspect that may well be ok for purposes of the French inheritance. (In the case of a minor child who inherits something, the legal guardian - i.e. a parent - would act in their interest.)
If you're not in France, I'd try contacting the French consulate in the UK to see what they can advise.
Cheers,
Bev