Marital tie is not yet finally tied. Marriage is not yet performed. So no legal relationship, as husband and wife, between you and the boy, is created. And therefore no rights and obligations in law as married partners arise against each other. This, to me seems to be the best factor on your side.
If from the heart of your heart, you have come to this decision to call off the marriage, then it is advisable to follow it.
As regards legal consequences:
You have stated that 1) Engagement expenses were borne by your family, and 2) You are ready to return every gift they offered.
So monetarily there does not seem to be much damage / loss to them. And on this count there does not seem to be chance of litigation by them against you.
But if they take this (your calling off the marriage) as insult, humiliation, mental harassment etc. then they may get hurt and in that case they may file a suit for compensation on account of these factors. (A hurt person may go to any extent of filing reasonable / unreasonable, civil / criminal, maintainable / non-maintainable case, which may unnecessarily result in harassment to you)
But this depends how they take it. Their understanding, maturity plays a role here. If they and their family members are matured enough to understand that it's the girl's decision, and that however against it may be to them and hurting it may be to them, they must respect that decision, then there is no chance of any litigation.
The best way to handle this situation is to tackle it with the help of understanding elders in your family (hopefully they are supporting you in your decision) and the mediator, if any. Let the situation be tackled in such a fashion that it doesn't hurt them, or humiliate them. These measures can be taken to avoid any future litigation.
Hope this helps.
All the best to you for your future.
Adv. Nahush Khubalkar