It is the value of your father's estate - his property, money, personal possessions and so on - that determine who inherits.
If the total value is �125,000 or less, his wife inherits everything.
If the value is over �125,000 his wife gets �125,000 and a life interest in half of everything over this sum.
His children - you and your sister - would be entitled immediately to half of everything over �125,000 and the other half on his wife's death.
But it is a very complicated area - on the face of it I would say there is nothing in law unless you are entitled to a share of the estate as mentioned, but you could speak to his wife and try and negotiate.