Be horrible to everyone. Dont take off your coat, flirt with the maid, be an ass at dinner.
Two items should be a pair of your sister's underwear and your old fencing sword.
Do not place the panties anywhere. Keep them on your person.
Make sure you bring up the inheritance with your father.
You don't have to do all these things. Just a few will arouse suspicion because you are already, by nature, the most likely to do the deed and the only one without a clear reason to be in attendance.