Right, when you mother died her share would pass automatically to your father if the above mentioned entry was not there. He would then own the property outright.
If you brother has paid off the mortgage then they may have executed a deed (declaration of trust) stating that he is entitled to a proportion of the proceeds of sale of the property. If there is no such deed or other document in place then there is obviously more doubt over whether he paid the mortgage off as a gift, or whether it was intended by all parties that he should receive a share in the proceeds of sale of the property.
If you brother and father agreed that he would receive a share of the property then that is probably binding, but your brother could potentially face enforceability problems if you father passed away and this was not expressed in writing anywhere.
If your father has a Will and you are not written in as a beneficiary then you may be able to make a claim against the estate under the Inheritance (Provision for Family and Dependent) Act 1976. You must make the application within 6 months of the date of the grant of probate in the estate and you will need a solicitor to do it for you.
The Court will consider what is a "reasonable financial provision" for you and will take a number of factors in to account in determining what is fair (eg. size of the estate, other claims, resources and needs of other family members/dependants, responsibilities the deceased had to you), each case turns on it's own facts and there are no hard and fast rules to work out what you are entitled to.
Without knowing what your brother and father have agreed it's all speculation, you might consider asking them together just for the sake of clarity. If there is doubt when your father passes away then it could result in an acrimonious probate which is costly and stressul.
Try to approach discussions dispassionately and constructively if you do.
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