37 years old and just finding out

Hello all, I am 37 years old and just found out I am adopted. I questioned my mother when I was growing up because I didnt look like anyone but she reassured me that I wasnt. I let it go for a very long time but honestly never felt much of a bond with her. My father, on the other, hand and I were very very close. Unfortunatley, he passed away 13 years ago. My mother suffers from Alzheimers so asking her about it as this point probably wont do me any good. About 5 years ago, a niece of mine ran into an old family friend and she inquired how "the little girl they adopted" was doing... I asked my mother at the time as well as my two sisters and they denied it. My "parents" as well as sisters and brother are much older than me. My mom is currently 91 and my father passed away at 81 as I mentioned before 13 years ago. My brother is living in Santiago, Chile now but has been in Florida visiting his son, my nephew, for the past week now. He finally admitted it to me but claims to have no details. He has no idea who my birth parents are and said my parents made sure they were listed as my biological parents at all times. Im wondering if there is a way to research all this myself. My birth certificate is for a "U.S. citizen born Abroad" my parents were in Nicaragua at the time of my birth (1976) but were already U. S citizens. How Can I uncover any information regarding all this? I am thinking of having DNA analysis only to break down my ethnicity. Thanks