My sister won't let me get to know my nephew?

I got a new car and my sisters boyfriend asked if he could buy my old one by paying it off while he drove it. I agreed he could and pay it off by paying off $50/fortnight which he did twice then stopped. After 3 mnths l reminded him about the agreement but after a mnth still nothing, so l reminded him again, which he ignored, so after another fortnight l said that if he didn't start paying l would have to take it back so l could sell it. After a fortnight of still seeing no money l took my old car back. 3 months after this my sister gave birth to their son and after spending a day with my sister and her new baby her boyfriend came over and said l was never to see their baby (my nephew), or hold him and that if l did l better watch out. All this because of the car. If l'd known this would have been the result l would have let them keep the car. I guess l just didn't want to be walked all over. I've never hurt anyone and never would, so why my sister would go along with him is beyond me. It just really hurts. How do l go on from here?