It is not straightforward. Joint wills are never easy.
How did your parents own the property? Joint tenancy or as tenants in common?
If it was joint tenancy your mother automatically owns the whole house and can do with it whatever she likes.
If it was tenants in common your father's will will apply to his share of the property; and your mother can do what she likes with her half.
If it was tenants in common and your father left his share to your mother, your mother now owns the whole and can do with it whatever she likes.
Assuming your mother has the right to will the whole property, she can leave it to one sibling, the cats home or the next door neighbour if that is what she wants to do. Fairness doesn't come in to it.
The exception is if you are financially dependent on your mother at the time of her death, then you may be able to make a claim.