Are nieces of sisters related

Some nieces of sisters are related to each other and some are not.

Your niece is the daughter of your brother or sister (your siblings). If you have more than two siblings and at least two of them have daughters, those daughters are your nieces and they are first cousins to each other.

You can also have a niece who is the daughter of a sibling of your spouse. If two sisters have nieces who are daughters of their spouse's siblings, those nieces are not related to each other (unless the spouses are related to each other, as when two brothers marry two sisters)