Your relation to your sisters daughter?
You would be her Aunt or Uncle. The child would be your Niece.
What relation is your sisters daughter to you?
your sisters daughter would be your neice and you will be there uncle or aunt if you are a boy or girl
What is the relation between my mum's sisters daughter and her husband to me?
Cousin and cousin-in-law
What relation is your sister in law's brother's son to your daughter?
The son of your sister-in-law's brother is not related to your daughter.
What relation is your mother's brother to your daughter?
Your mother's brother is you uncle and your daughter's great uncle. Your daughter is his great niece.