Is this correct? 家族に中で背が高い誰ですか。
You would use の instead of に here (linking two nouns), so 家族の中.
You also want to insert a superlative (phrase that means "most"/"best"/#1). One example is 一番（目）. There are many others, like もっとも, だれより（も） (less common), etc.
So something like:
For some basic examples of superlatives, see this page.
- Generally speaking に is used to mark a place in space or time, you'll want to use の in 家族の中で.
- i-adjectives(形容詞) are followed immediately by a noun, therefore 高い誰ですか which is followed by a "question word" is not grammatically correct.
If you want to be polite use this:
if not, the answer above is best.