Is there a gender neutral term for people who are first born in a family?

For example, if a mother, father, and older sibling are all the first born to their parents, is there a term that describes all of them?

A quick check in google has shown me 長女 and 長男, as well as 初生的 which I'm not clear about, but if I wanted to say that "your family members are all X's" and use the correct Chinese word for clarity and to avoid any ambiguity, is there a word or simple expression for this category of people within a family? Or even without being within one family?

I'm not sure if this should be asked separately or not, but if there happens to be a word or phrase for the only person in a family who is not first born, that would also be helpful.