Legal advice on wedding called off

My younger sister wedding was planned on Dec 5th 2015, which didn't happened. I am explaning the scenario 1. When my father contacted groom mother for alliance she said she want that first his son talk to the girl. Then everyday that guy used to chat with the girl over phone. The alliance discussion proceeded and guy mother gave list of demands. Somehow my father managed and he aggreed on every demands and then Engagement date got fixed. Engagement happened on 30th July 2015. 2. After the engagement completed now groom mother startedputting some more demand related to gold and gifts. Somehow my father managed to give all these by taking loans and he aggreed on everything. Meanwhile this groom who used to chat with my sister everyday before engagement now he dont even say Hi to my sister over mesasge also.We ignored things as we thought he might be busy. 3.Whenver any of the family members used to call him he just ignored our call. Just 3 days before the marriage when I called him, he ignored mine call also. At the same time groom mother also gave so many pressure to my parents related to gold and gifts. 4. Frustrated with all these my father cancelled the marriage stating that even Groom is not respecting the bride who will respect in their family. 5. Now when my father asked them to give back all the gifts including 7lakh cash and 6 lakh jewellery they refused that and saying they dont have anythings and you are telling lie.They even told my father that if you take any legal action I will put "Manhaani" case on you and you have to give 10 lakhs to me instead. Please advice sir how we proceed we come from middle class family and we dont have that much legal knowledge.