Mutually breaking engagement after marriage

My brother got engaged 10 months ago and marriage was supposed to happen next month. Amidst the Covid situation, we thought of having a simple reception but suddenly Bride's father called us and ordered us to book a hall for reception as he'll invite 50 odd people to our home and threatened to call off wedding we we didn't book it. Also he said for marriage he is gonna invite 1500 people. We were scared because as per the state's guidelines only 20 people should be allowed. We tried explaining him but he was noy ready to listen. We even suggested postponing marriage few months later as pandemic might ease. He was also not ready for it. Then we said it might not be possible now and we can seperate, he also said fine and it was mutual. But now after few weeks, their lawyer called us and said they are gonna send a legal notice. Could you please advice what can we do here? We have proof of all the call recordings where he threatened us saying he'll invite 1500 people and he was the first to say that he'll cancel the marriage we have call recording on that as well. Kindly advise.