If you are your sister's biological brother or sister, you can call your sister's grandson your "grandnephew". A grandnephew is a nephew's son or daughter's son, which is your nephew's child. If you are uncomfortable with this title, you can negotiate with your sister's grandson and his parents to choose a more friendly and convenient title, such as "Little Baby" and "Little Cute". Or if your sister's grandson is still very young, you can directly use his name or nickname, such as "Xiao Ming" or "Little Bear". In short, any name that can be clearly identified and comfortable is acceptable.