Your aunt-in-laws nephew is your nephew-in-law.
Although, a lot of people would just say nephew or niece instead of nephew-in-law or niece-in-law as niece and nephew are more informal and easier to say.
Recommended for you:
Family Relationships in English And Phrases About Family
Most commonly used Phrasal Verbs about Family
Collocations with Friend!
How to say BROTHER’S WIFE in English?
