Your father, as the son of an affected person (your grandmother) has a 50% chance of having inherited Huntington's disease.
So technically, you and your brother both have a 25% chance of inheriting it yourself.
Generally, the age of onset in children anticipates that of the affected parent: the age at which his grandmother showed the first symptoms should eventually be taken into account.
Of course, if your father hasn't inherited the mutated gene, neither you nor your brother can ever inherit it yourself.
In any case, beyond the technicalities, there are many variables to take into consideration, depending on the specific family history.
If you need to know more, why don't you call us and make an appointment at one of our free clinics? It is also possible to have some more informations via zoom or Skype, if you prefer.
We remain at your disposal for any further information.