A group of nieces is commonly called a Group. Other collective nouns you may see include Bunch and Gathering.
Collective nouns are specific terms used to describe groups of people, animals, or objects. In the case of nieces, a collective noun is not commonly used as nieces are usually referred to individually. However, one possible collective noun that could be used to describe a group of nieces is a "generation of nieces". This term emphasizes the familial connection between aunts or uncles and all their nieces collectively within a single generation. Beyond this, there is no widely recognized or established collective noun specifically for nieces. As such, when talking about nieces, it is customary to refer to them by their individual names or use general terms like "group" or "set" of nieces.
Known collective nouns for Nieces include: