Conditionals are not tenses!
@plethora is right, but not for the reason they state. Conditionals express some kind of counter-factual assertion. In this case, your sister doesn’t know how far from home you are. In this situation, the most common pattern is, as you correctly use, the past simple form of the verb – “knew”.
What is, in fact, confusing you is the form of the verb in the clause reporting what your sister does (or doesn’t) know – it is possible to use either “was” or “am” here, but not “were” (as there no counter-factual statement being made) – you are far from home; this is not the counter-factual statement.
The issue is one of whether you should use the present or past form of the verb in reporting clauses. Generally speaking, I suggest to my ESL students that one uses the past when there is some degree of doubt over the veracity of the reported statement or it relates a past event.
Think about it – remove the condition, and which one seems more natural?
> She knows how far from home I was right now.
or
> She knows how far from home I am right now.
Compare:
> She lied to him about how far from home I was.
and
> She lied to him about how far from home I am.
Cf. epistemics and modality