"N'avoir ni tante ni oncle."
However, this sounds a bit formal and unnatural in everyday conversation. Here are some more natural ways to say it in French:
More informal:
* "Je n'ai pas de tante ni d'oncle." (I don't have an aunt or an uncle.)
* "Je n'ai ni tante ni oncle." (I have neither aunt nor uncle.)
More specific:
* "Je n'ai pas de tante du côté de ma mère/de mon père." (I don't have an aunt on my mother's/father's side.)
* "Je n'ai pas d'oncle du côté de ma mère/de mon père." (I don't have an uncle on my mother's/father's side.)
Choose the phrase that best suits the context of your conversation.