I need a word that fits this description: something that completes somthing else.

Do you want a noun? Maybe something's compliment or match.

Like I want to say she completed the family. She was the _________.

I need a word for the blank.

http://www.answers.com/complement

(not compliment)

So I would say she was the complement?

You can, yes.

To find a word that fits the description of something that completes or enhances something else, you can consider words like "complement," "supplement," "augment," or "enhance." These words are often used to describe something that adds value or improves upon another thing. Additionally, you can use a thesaurus or search for synonyms of the word "complete" to generate more options.