It's understandable that you may be a bit shy but it sounds like your twin has had no problems and it will likely be the same for you. Most girls begin their menstrual cycle around the age of 12, but a period can appear as early as age eight. Talking about menstruation with your mom may feel awkward but it's an important discussion, and a major milestone in your life.Your mom can help answer your questions and help you with any problems you may experience as you start having regular cycles. She can help make this easier and that's one of the best reasons to find a way to calm your nerves and just sit down and have a nice talk with her.
Good luck to you, hope it goes well!Pat