1) you are in possession
2) the flat is in your name.
3) loan is taken by you
4) benami act is also applicable
5) your parents and other family members are staying at your mercy in the house.
Result: you dont need to worry about the ownership. No one will be able to take your flat from you unless you let them. Do not give up possession at any cost. remember this. If they pass threats lodge police complaint. send notice to your sister and brother in law to vacate. SHe is not your responsibility. Though morally and legally you need to maintain your parents but i am sure once your sister and brother in law are out of the house they will not be able to influence your parents's judgement decision making.