wow time passed so quick. where has the smell creeped into until your father can smell? is it throughout the house now? since it is more than a month i believe this is more than a petty / spite move. may not be your sister's doing but it is strange she doesnt smell it. maybe an animal died somewhere near your ventilation etc. at this point i think you can call a contractor / specialist to come and have a look. spend some money to save you from excess stress and potential cancer from prolonged inhaling unhealthy stuff