A sibling's spouse, children and grandchildren will all have different relationship names. A sibling's spouse will be the person's brother-in-law or sister-in-law, and the sibling's children will be the person's nieces or nephews, depending on the gender of the child.
A sibling's grandchildren will be the person's grandnieces and grandnephews. As more generations are added, the prefixes "grand" and "great" are used in combination with numbers to indicate how far removed the person is from the aunt or uncle. While specific genealogy terms can extend well into five or six generations, the terms "niece" and "nephew" are often used in normal conversation to refer to any of the sibling's children or grandchildren.