Five marks requires five main points with detail and development. In simpler terms, make each new point about your family member, and then say something interesting about them. That could be their job, their appearance, their personality or something you do together. To get higher marks, put in a special sentence structure too.
eg: 我家有五口人:
我有一个妈妈。 她有黑色的头发。 她是一个律师。
我有一个爸爸。 他对我很严格,总是想我做作业。
我有一个弟弟。 他是十五岁。 我喜欢他因为放学后, 我们一起打网球。
我有一个哥哥。 他是大学生所以我常常见他。他很好学。