An Aunt.
sister friend
Aunty and Uncle.
Aunt
You would call your father's sister "பெரிய அந்தை" (periya anthai) in Tamil.
philapple Uncle
Sister in law
Your mother's sister is your aunt.
Brother-in-law
cousin
The relationship of the sibling of your sister-in-law or brother-in-law to you does not have a name in English. You could call them your brother- or sister-in-law if you want, or you can call them the brother or sister of your brother- or sister-in-law.
The English language has not special term for the husband of your sister-in-law. You woudl call him your "sister-in-law's husband."
sister in law