If he has made a Will then his estate would obviously pass in accordance with that.
If he marries his partner then the first £250 000 (plus his personal possessions) would pass to her provided that she survived him for 28 days. Half of the remaining amount of his estate over that would pass to his children (you and your brothers/sisters). HIs partner/wife would have the right to the interest on the other half until her death at which point it would then pass to you/brothers/sisters.
If he makes a Will leaving his estate to his partner 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/dependents, 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.
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