Not sure if im adopted?

I think i might be adopted, 1. the night before my sister got married she said to me i don't have to be your sister anymore.2. my dad his cousin her grandaughter and i worked in the same place two years ago and she mentioned about adoption in the family i was slighly taken aback, my mum always made reference to my Aunt if you were living with your aunt ***** etc, now my dads name is on my birth cert but i know from experience anyone can put there name on it!!!now im not a teen ager im 48 also both my parents have passsed and i asked my brother but he do's not know , i have tried to acess my mothers medical records to see if there is anything in relation to my birth but to no avail, i always did feel different, and if i am adopted its a family adoption just don't know were to get answers from ..