Twins meeting later? At diff age?

Okay I post to had a twin but my moms egg or what ever the baby grows in didn't full form into a egg so anyways on my 16th sept 8 my sis had her baby at the same time and day I was born and my friends say it's my unborn twin but she does cling to me more than others and I don't see her maybe once a month. Is it true or not she's my unborn twin is weired bc my sis was due in nov.