How big is your family?

你家有多大？

How many people are there in your family?

你们家有几口人？

How many do you have?

你有几个兄弟姐妹，

Do you have any brothers or sisters?

你有兄弟姐妹吗？

I'm the oldest/the youngest/the middle child/an only child. 

我是最大的/最小的/中间的孩子/独生子。

My parents were not expecting to have me, they told me I was "an accident". 

我父母没打算生我，他们告诉我，我是一个意外。

We are expecting a new addition to our family in May. 

我们家在五月份将要添一口人了。

I come from a big family. My parents had ten children.

我来自一个大家庭，我父母有十个孩子。

I have five sisters and three brothers. 

我有五个姐妹和三个兄弟。

Conversation:

A: Are you married?

你结婚了吗？

B: Well, no. Why do you ask?

还没有呢，怎么啦？

A: I thought I saw you with your son yesterday having lunch. 

我想我昨天看到你跟你儿子一块吃午饭了。

B: Oh, that wasn't my son. That was my nephew. My sister's boy. She also has a daughter.

那不是我儿子，那是我侄子，我姐姐的孩子，她还有一个女儿呢。

A: If you don't mind me asking, why don't you get married?

如果你不介意的话，我能问问你为什么不结婚吗？

B: Well, I think that's kind of a personal question. Maybe I haven't met the right guy yet, or maybe I'm comfortable being single.

我想这是个私人问题，可能是因为我还没有遇到合适的人吧。或者可能是因为，单身的生活让我感到很舒服。