You say the will has "gone to Probate", but now the probate office have told the executors they must wait six months. It sounds to me like the niece has lodged a caveat. This prevents the Grant being issued. The executors should enter a warning to the caveat to move things on. Unless the niece enters an "appearance" to the warning, her caveat will be dismissed and the Grant issued, which means the estate can be administered. If the niece enters an appearance, the executors should issue a summons for directions to move the claim forwards.
Can you just answer these questions please? Was there an earlier will? If so, do you know it's terms? Exactly what is the relationship of the "niece" to the auntie - "by marriage" seems to indicate she is not a blood relative. Has she indicated on what basis she will contest the will?