I feel like I have a twin?!!?

Everyone feels like they have a long lost part of them missing. It's not a twin, it's just a lack we all feel in being human. Completely natural. You can talk to your mom about that lacking part, but the whole twin thing - you can ask, but she'll confirm a no.

While it's highly unlikely if your really bothered about it mention twins around your mom or watch the parent trap or something like that and ask how weird would it be if I had a twin. Watch her reaction unless she is like a spy you will see some sort of response like a twitch or a tight smile

you are wrong. you dont have a twin. its something else that makes you feel that way (lonelyness, not being satisfied with your life, could be anything)