Second line needs to have "has she graduated" to be correct. Since the phrase in your question presumes a comparative, you do need to provide one. So, as others have said, something like "not as close to your sister as you are to your brother" for example would work. Or "you are not as close to your sister as you used to be". In any case you do need the second part of the comparison.

You are not as close to your sister = You have a good relationship with your other sisters, but you do not have a good relationship with the youngest sister. In this sentence the purpose of "as" is to compare the two relationships mentioned.

In this sentence "as" is an adverb used to express extent or degree in comparing two things. Mike is not as close to his youngest sister as he is to his other sisters. The relative clause (as he is to his other sisters) is implied from context given with the earlier part of the conversation.