Meeting an unknown half sister

This story starts in a very small town in Spain. 45 years ago my mum lived in this little town and was married to my dad, but had an affair with an old love. A couple of months later she finds out she's pregnant and she doesn't know who the father is. She shares her doubts with her lover but decides to believe that this baby, who would then be me, is her husband's (I didn't know anything about this). This lover of her, who now lived in a different city, 650km away from my mother's, gets married and has two daughters. I never heard of him or them. Never even knew they existed. One of the daughters, the oldest of the two, we'll call her A, decided when she was about 25 to spend a year abroad learning English in NYC. There she lives in a shared apartment with other young girls, one of them being, coincidentally, one of my cousins, who had also decided to spend some time in NYC learning English, we'll call my cousin B. They became very good friends, but none of them knew they were somehow linked. A couple of year later, when they weren't living in NYC anymore, A addresses an email to many people she knew. The content of that email is not relevant, but what indeed is relevant, is that was still the time when people still used to write many addresses in Cc instead of Bcc, so everyone could see who were the other recipients of the message. In that message, A's father was included and also my cousin B. It turns up that my cousin B, my mum and I all have the same surname, plus it's a very rare one, and this catches the attention of A's father, who sees the surname of that once lover of his and asks his daughter about it. "Who's that girl with that surname? where do you know her from", he asked. A innocently tells him this is a friend she met back when she was living in NYC. He asks her to find out whether this B friend of hers knows a person called like my mother. A calls B and finds out she indeed knows a person called like this, it's her aunt. A's father doesn't tell her much more than "wow, what a coincidence, I once knew someone called the same way!", and that's all. But apparently he's quite restless because a few years later, when A turned 40, he decided his present was going to be the following: Telling her a true story that she doesn't know: She has an older sister, that's me, and I'm actually the cousin of one of her best friends. I was completely blind about all of this: I only found out, quite uneasily, when A, suddenly asks me for friendship in Facebook. This was a couple of days after her father had told her that story. I accepted her because she was a friend of my cousins and at that time I was promoting a book that I had published and was doing a lot of advertising in social networks. She started messaging me for no reason, maintaining just small talk, until three months later, she tells me she has the key to my life, but says nothing else. She said I should get in touch with my mum and ask her about a man's name I had never heard before. The rest of the story I'll rather keep it to myself, because it's full of extremely sad moments and has nothing else to do with coincidence. But the rest does and I think it's quite awesome.