Undecided on when to have "The Talk" (Lengthy)

Hi all I am in need of some advice. My situation is a bit different but not yet uncommon. My Husband and I adopted a child from a relative at birth. My child is now 7 years old and is not aware. The relative that we adopted from had a child previously that she no longer has custody of. She also has taken so many drugs that it has affected her ability to take care of herself and she is currently living with another relative. She is almost 30 at this point. I have spoken to other mothers that have adopted within the family and some that adopted a child from a previous relationship and I am getting both positive and negative responses to having the adoption story being shared with the child. While there are people around us who are aware of the adoption, this isn't a known fact to most of the outside world as our child does look like both my husband and I. We have recently decided to tell our child about his "birth story" since the age is appropriate now and we were going to wait until Summer break so we would have time for any necessary time for talks or whatever we needed to ensure that the emotional well being of our child is in a good place but after a conversation with a friend over the weekend; I am not feeling this may be a good idea. I was told that once my friend told her child that she wasn't her biological mother at the age of 8; that she could tell that in her eyes she was crushed and things changed. Her child is now 18 and is acting out after her biological brother contacted her and there is little to no communication as to what struggles she is facing right now which does scares me beyond infinity. With us having a relative adoption we also have the task of explaining, at some point, the family tree. I don't know that is it beneficial to our child to know that uncles/aunts are really grand parents, grandmothers are really great-grandmothers and then there also a biological sibling involved. We have contact with our child's sibling because she is our cousin so I feel as though we will affect more than one life by telling our child right now and I know that neither of the biological parents want us to say anything but I also don't want to hide or lie to my child. I also have some fear now that the biological mother is 20 minutes away from my home and if my child knew who she was; I wouldn't trust her around my child. I don't know what she is capable of and if I tell my child who she is; how can I tell my child to stay away from her should she come around? In my head, it's almost better that she stay a distant relative that my child has seen like 3 time in their life. My child doesn't even know her so I'm so afraid that it would be too confusing and will do more damage than not. I am looking into finding a family therapist to get a professional opinion this week but I thought it would helpful to talk to someone who is an adopted parent or someone who has been adopted. I appreciate any stories of experience and/or advice. Please no judgement on our life decision. I want the best for my child; whatever that may be.