A wedding in the family and that too her sister’s is reason enough to try to patao you so that they don’t loose face in society. It is obvious that their relatives and friends are not very well aware of the situation. Don’t give in at this stage. You are right and stick by it. You will benefit in the long run. Return her stuff when she has signed all papers. She has to build trust and there is no getting around it. The pressure to comply will build up in the days before the wedding. So be prepared.
A good suggestion may be to stop calling her for a few days. See if she calls you. Talk to her, but no promises. Let her chase you. If she is serious about wanting to build a relationship with you, she will sign. Otherwise it is idle talk which is not going to lead anywhere. When a marriage breaks and the legal fall out is sec 498a and DVA, it is pointless trying to talk trust. Trust has to be established by way of legal documents. Otherwise it is all a fallacy.