I am mexican/white with very curly hair

My brother an I have very curly hair, almost an Afro. We're a mix of white and Mexican. The only problem is that no one in my family has the same hair as us. All of my family has naturally straight hair. The only person who has had curly hair was my late great grandmother on my Mexican father's side. If My brother and I got it from her, it would mean our hair skipped two generations! I know that We were never adopted or had a different father, so how could this happen?