revenge is sweet.
watch the austrian film “Revanche”.
do not let the woman get away with it. pester your missus till she screams and shouts at her sister. in fact i’m surprised that you do not mention that your missus is even a bit pissed off with it all. i’d be tempted to take the money from your missus if she doesn’t see it as her problem/responsibility.
is your FIL still around? how does he see it?
enter her phone number in every bit of junk mail you recieve.
enter their address on every mailing list you can.
complain to the council (anonymously!) that they are bad nieghbours and let their dog crap in the street.
give their address to the local jehovas.
really piss them off.
it won’t get your money back, but in the meantime it’ll give you some pleasure.