Born April 30, 1990 in MI

I'm looking for my birth mother and sister. I have no information as to where in Michigan I was born or in what hospital. I was told that my parents went around to high schools looking for pregnant teens who were looking into adoption. Not sure how true this is. My brother was also adopted, although from a different mother. She tried to reach out to my parents to see him when he turned 18, but they refused and never told him about it. So I fear my parents might hinder any attempt at finding mine. The only help I have is from my Grandmother, who told me I had a sister who looked just like me as a small child, and that the mother had to give me away because of financial issues. No other information was provided and I did not press further. I am afraid that any attempts at official searches will result in my parent's notification. I am at a loss at what to do. I would rather not post any person information or names here, so email if you need them.