Should I listen to my mom?

My mom just told me about a baby she gave up for adoption in '69.(I am 33) I asked my mom if she knew where she was, ect. she said no and that she had left info for her to find us if she started looking. Well, As you can imagine..Wow! I have a sister.I was the only andyougest sibling girl with two other brothers with my mom and dad. I would have an older sister. My mom does not want me to try and find her. She says that she might not know and it isn't my or her place to tell her or if she does know she can contact us. What do y'all think? Any imput would be helpful. Thanks.