Received a set of Blank Papers as notice. What to do?

Dear learned experts, My sister has been married for almost 2 years now. She was deserted by her husband soon after the marriage, as he fled abroad and her in-laws started traumatising her for dowry, subjecting her to immense amount of agony and harassment. Husaband ceased all contacts with her and as it became unbearable, she started living with us for past 10 months. Now having exhausted all our options to sort out the matter through talks as they are adamant at their demands, after much deliberation and having run out of our patience, we have filed a complaint in CAW cell stating all the facts. Ever since her in-laws and their relatives are trying every trick up their sleeves to pressurise and threaten us to take back our complaint and remove their names. Now, alongwith these nasty developments we received an envelope today, addressed to my sister, that mentions our home address incorrectly (but was still delivered as the postman is quite old and familiar with other details of our address), reads SPEED-POST, and bears stamped details of the lawyer who seems to have send it. My sister received it assuming it might have something important. There are no other details on the envelope whatsoever except the Registered Post sticker on the back that has details about the weight of the package. As soon as we opened it, we found it contains a set of blank papers. We immediately showed it to the delivery-man, counted the pages in front of him and made a video of it too. He didn't allow us to leave any remark on the delivery challan though. We even contacted the post-office, but they also denied saying no remarks are allowed on the delivery sheet except the name of the receiver and the phone number. It appears to be a clear case of some kind of fraud or mischief or a tactic to bog us down by my sister's in-laws. I instantly searched for the details of that lawyer whose name was on the enevelope, and I was shocked to see that he is holding a very influential position in one of the BAR Associations of District Courts of Delhi. I immediately called the President of that BAR Association and he told me that it's a common malpractice nowadays and that I should file a Police Complaint and also told me not to disclose that he advised me to do so. I, then contacted the CAW Cell and also the concerned courts. They advised me the same. Now, as you can see that under the garb of this tactic this process can be blatantly misused against us by my sister's in-laws to address their ulterior motives. So, my humble query is what is the way out to nip this malafide intention of theirs in the bud and also what action can be taken against that lawyer to excise his unethical practices and any further attempts of threatening, blackmailing and doing fraud with us to weaken our case and thereby save ourselves from any unwarranted, unforeseen developments that may put us in trouble. Moreover, since this lawyer happens to be a member of BAR Association, can the BAR not pull him up and punish him accordingly post a departmental enquiry? Please show us the way. Thanking you all in anticipation!