What is a term for the family unit led by an aunt or uncle?
You can look at it from the view of your contemporaries and say, "I'll be visiting my cousins"
However, this still leaves the question of whether they are relations of your father or your mother. To include grandparents, aunts, uncles and cousins (and clarify) you could say:
"I'll be visiting my mother's/father's side of the family" or 'my mother's/father's kin'.