how to prove hat he did demand for money
if he demanded for money or anything else after marriage then things get a little difficult. if any of his demdns were ever met then good...you can prove that as mentioned above...if none of those demands were ever met then...this exactly where your parents and relatives get into the picture with their witness and ofcourse cross-examining of your brother-in-law and sister-in-lw would also help...you can also try to prove that he was indeed need of money then...like say may he was in debts...if he wanted to buy a flat or a car or something...he would done some research...you can gather evidence for the same and question him as to why was doing all this when he never had the money to buy it...from where was he planning to pool up the money for the same...however its difficult though...it all depends on how he handles himself with these kind of questions...
but trust me if has done the mistake of harassing you for dowry he is bound to do mistakes to save his ass...theres a saying...'ek jhoot ko chupane ke liye 100 jhoot bolne padte hain'...they become overly defensive and end up doing a mistake...so just relax...if your allegations are not fake and its a genuine one then its him who needs to worry..not you..