Why don't you outright tell your cousins and other relatives that that bothers you, and that you'd appreciate it if they referred to you as she? That's a lot easier said than done, I know, unless you're an assertive person (I happen not to be, so I completely understand if you're not comfortable in being so direct). Otherwise, that sucks. There's not much you can do except take it and know deep down that you're a woman. Again, I imagine that's easier said than done. Just trying to offer some advice...
One of my best friends was a MTF transsexual. I met him online when she was 16. She didn't know of her transsexuality then. Then, when she turned 18, two years later, she told me how she felt, and I was very accepting, because I was always very accepting of gays, transsexuals, and other groups that are often frowned upon by society. I couldn't shake the habit of referring to her as heor him, however. It bothered her immensely, even though I immediately corrected myself afterward. We don't talk anymore, but even when I think about her, I still sometimes think of her as a man, or I'll say to myself, "Haha, he was so funny," when I should be thinking, "She was so funny." My point is, there's a difference between deliberately referring to someone as a male when she's female or as a female when he's male, and slipping up because you've known someone for a really long time. Is it possible that your mom and your cousins and other relatives are just slipping up? Or are they doing it deliberately?
