Hitler never had any children, so therefore no grandchildren or great great great great great great grandchildren either. And even if he did, Hitler did not live that many generations ago.
Step-great-granddaughter
Your brother's granddaughter is your great niece and you are her great uncle if you are a man, or her great aunt if you are a woman..
Your sister's granddaughter is your great niece and you are her great aunt (if you are a woman) or great uncle (if you are a man).
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your grandfather's brother is your great uncle. Your great uncle's daughter's daughter (his granddaughter) is your second cousin.
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
Example: Great-great-great Granddaughter Katie
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
Your sister is her great grandaunt.
Your brothers great granddaughter is your great great niece.
Your sister's daughter is your niece. Your sister's granddaughter is your great niece Your sister's granddaughter's daughter (your sister's great granddaughter) is your great great niece. You are her great great aunt, if you are a woman, or her great great uncle, if you are a man.
Step-great-granddaughter
granddaughter
Your great uncle's granddaughter is your second cousin.
The word is "bisnieta". Note that "nieta" alone means "granddaughter".
Your grandmother's brother is your great uncle. Your great uncle's son's great granddaughter is your great uncle's great great granddaughter. Your great uncle's great great granddaughter is your second cousin, twice removed, and is your daughter's third cousin, once removed.
Your great aunt is the sister of your great grandparent. Her granddaughter is your second cousin and her great granddaughter is your second cousin, once removed.