As I said if your father in law choses to apply to the Court for an Order for sale then it will be granted.
With regard to how the shares would be divided between you that is harder to assess
It is clear that it started at 50/50 between your husband and his father.
Equally it is clear that you brought no capital to the property when you were joined onto the title.
That being the case it will be for the court to decide what the division should be
The work that you have done extending and improving the property will be useful - but there will no doubt be various valuations required.
The fact that the Will allowed for the property to pass to you and your husband is not relevant given his remarriage I am afraid
The £9,000 represents very roughly the difference between 35% and 40 % of the equity.
It is always important to be clear about what you are actually arguing over in cash terms
I hope that this is of assistance - please ask if you need further details