Gender neutral term for family members who are all first born? 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 translating "first born" to traditional Chinese using google shows 長女 and 長男, as well as 初生的 which I'm not clear about, but if I wanted to say that "other 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.
Speaking from personal experience (as a 長孫 on my paternal side):

老大 would be the most common gender-neutral term used within the family. Very common.

長子 can refer to the one oldest child of brothers and sisters, but there is quite a lot of variation in use between families, and can refer to the "eldest son" or the "eldest child". To be clear, I'd often go with "長子長女" as a phrase.

A more neutral circumlocution would be 最大的 which is spans formal and informal, in-family and out-of-family use. It defaults to referring to age (not height / width!), but if you want to be specific, you can use 年紀最大的 with "age" included.

There isn't any two-syllable word for the youngest that my family uses, although 老小 and 老幺 I have now seen on searching the Internet. 最小的 is the usual gender-neutral way of expressing it.