How come some family relationships have individual gender specific terms and collective terms, while some have only one?
The adults who are responsible for conceiving, birthing, and raising you are collectively called "parents" and individually "mother" and "father".
The people that you conceive, birth and raise are collectively called your "children" or individually "daughter" or "son"
Your parents's parents are collectively called "grandparents" or individually "grandmother" and "grandfather" (amongst other affectionate terms). We also can continue with greats, great-great, etc.
People who have the same parents as you are collectively called "siblings" and individually called "sister" and "brother"
These relationship are directly in your family tree. But other relationships that are "one-offs" lack either the collective or gender-specific term.
Your parents' siblings are individually called your "aunt" and "uncles", but (as far as I know) there is no collective term to refer to your aunts and uncles together.
Your siblings' children are individually called your "niece" and "nephew", but no collective term exists for this grouping.
Your parents' siblings' children are your "cousins" but there is not gender-specific term for your female or male cousins.
This has always interested me, and I have coined my own phases for this missing labels: I have auncles, fecousin, macousins, and niecphews.