Real sister, half sister, or step sister
your sister
She would be the same relation to you as she is to him.
They would be cousins, once removed.
There is 2 answers your mothers brothers children or your dads sister;"s children
sister in law.
If your niece's is by your sister but the brother is by your sisters new husband & another woman there would be no blood relation. If not then he's the nephew
Your sister's sister is your sister.
You just call her Sister in law.But however correct term is co sister in law
It says what relation are your brothers nieces to you.... It could be your child or your niece also causes you can have two brothers and it may be the other ones kids. Or of course it could be your brother's new wife kid by another man making her no relation.
She would be your step-sister
No relation whatsoever. You will be an aunt or uncle to your sister's child and your brother-in-law's sister will be the child's aunt, but that's as close as it gets.
Your mothers sister is your Aunt
Jonas Brothers don't have a sister