What does the one Olsen twin know about Heath Ledger's death?

Is it Mary-Kate? I get them confused even though they have different hair colors! I read something about she may know something about the drugs that he took. Was she "partying" with him? Were they seeing each other? I heard she may have to testify. So, what's the deal? Didn't he die anyways of an overdose of prescription drugs that were prescribed to him and not of street drugs?