it's a trespass. you could call the cops. they won't arrest anyone but the conversation could make an impact.
you could tell the landlord and see if you're satisfied with the response
you could change the locks and deduct the cost from rent bit you have to give the new Keys to ll -- with the caution that you'll know where the sister git the new keys
I would say don't worry. do you have a copy of the envelope you mailed? if they did not provide forwarding address it was their duty to put forwarding into post office. mailing to last known address is proper.
you should initiate the small claims action for your full damages before they do
well who caused the fleas? a small claims judge would decide based on the proof? flea prevention is on your side ... a cat is against you. was there a pet deposit? did they try to address fleas with over the counter solution first, etc. .... without going to court? threaten court and settle for 50/50 split of costs?