Joint accounts are ordinarily subject to the standard rule of survivorship – upon the death of the first, the entire account passes to the co-owner absolutely. This is common and convenient for married couples but not so beneficial to parent and child, if the remaining child(ren) see the only asset of their parent pass by default to the one child whose name appears upon the account.
That is the situation you now find yourself in over the £70,000.
Joint accounts that are deemed to pass by survivorship “pass” outside of a will. It would therefore be irrelevant if your mother had made a Will gifting everything between her two children equally.
However, when one person alone puts money into joint names, there is a presumption of a Resulting Trust in favour of the provider of the funds.
(A Resulting Trust is a type of trust that is imposed by law. It returns the beneficial ownership in the trust property back to the settlor.)
As that is there case here, the funds were from your mother and she enjoyed interest from them, your brother does have a claim - not to a quarter - but to half of the funds.
However, the presumption could be rebutted if you can show evidence that your mother intended to transfer the beneficial interest in the fund to you. The burden of proof rests with you to show what you have described here about the fund being in lieu of past work done and the business partnership.
Under the laws of intestacy, if there is no surviving partner, the children of a person who has died without leaving a Will will inherit the whole estate. This applies however much the estate is worth. If there are two or more children, the estate will be divided equally between them.
In this case it will be half of the estate after deducting the £70,000 if you can provide proof as described for the joint account.
Technically, you brother is now living in a property half of which is yours. I doesn’t sound as though you want to force the sale of the house and make him homeless but, at the very least, you need to get the title deeds put into joint names. You need form AS1 for you as administrator of your mother’s estate to assent i.e.transfer the title into joint names with your brother.
www.gov.uk/government/publications/whole-of-registered-title-assent-as1/guidance-how-to-complete-form-as1
The it’s up to the two of you to come to some arrangement over how you each benefit as equitably as possible now from your inheritance, for example, whether he pays you some rent in lieu of the cash you have given him.
He needs to be aware that if you were to predecease him, then your beneficiaries (should you leave your estate away from your brother) could force the sale of the house unless you write something into your own Will that leaves him a life interest in your half.
That leaves you in a strong bargaining position at the moment as regards the £70,000 and getting some rent.
I hope that makes sense.