I did an abortion for the man my sister wants to marry, should I tell her?

My name is Ama (not real name), a 30-year old single lady working in a bank in one of Nigeria’s famous cities. About three years ago, I had a brief affair with a good-looking man. He is two years older and works in an oil company. We actually met while I was in the marketing department of my organization. Before long, we started seeing each other and even spent nights at each other’s place. I had great affection for him; he was a real gentleman with nice qualities. However, the relationship lasted only five months. It ended after I carried out abortion because according to him, he had no plans to settle down at that point. I felt used, dumped and couldn’t bear the pain any longer so I broke up. After completion of her National Youth Service last year, my younger sister travelled out for a Master’s programme in the United Kingdom, same country my ex was transferred to from Lagos in 2011. He had informed me about the promotion and I wished him well. We keep in touch once in a while, although no feelings attached. The shocker: My sister has always been telling me so many sweet things about her boyfriend and that she can’t wait for me to meet him. She obviously was in love…I was very happy for her because just like me, she had gone through so many heartbreaks. In December, she sent pictures of her and a guy together at a friend’s birthday party. At first, I thought my eyes were deceiving me; it was my ex! I recognized him the second I saw his face. As if not enough, I got yet another shocker in the third week of January. She said my ex has proposed and that they are planning to get married later this year. Honestly, since hearing this news , I have been very uncomfortable. Few days ago, I told my ex we needed to talk so he called. I expressed my fear and reservation of him courting my sibling considering our past. He said he already knows that I and his new love are related. That he found out after several conversations with my sister and also several pictures of us. He said he hasn’t said anything yet and that we should keep sealed lips, since we dated before they met in the UK. Initially I accepted his view but lately I have been miserable. I really wish to get this off my chest but at the same time I don’t want to hurt my kid sis. I am mad at myself, I am mad at life. What do I do please?