How much contact is balanced ?

I am a distant relative attempting to adopt the two children of my cousin. My cousin and his wife are seperated. They both are drug users and parental rights have been terminated. The children are now in the state system awaiting adoption. It is common knowlegde within the family that the birth grandparents also use drugs but I have no way of proving it officially. It is one of those dark family secrets. They were also physically abusive to myself and my cousins when we were little kids, so even if they cleaned uptheir drug habit, I would fear the b-grandparents' judgement & behaviors in reguard to kids. Prior to the adoption situation I had no adult-years contact with this side of the family as I chose to live a healthy and productive life. They have recently told me that they do not consider me family. In our home study my husband & I stated that we were not opposed to contact with the birthparents with the stipulation that they would have to be reformed - have gone through rehabilitation and proven stable & clean before we would allow a contact as we do not condone drugs or want to subject these children to the even more complicated emotional trauma of interacting with drug users. We did not bring up the issue of the birth grandparent's drug use but were told that they have been very vocal about being against our adopting these children. Since I am a blood relative I do not have the option of a closed adoption and we do want the children to benefit by continuing their relationships with the more stable people in our family. My question is this: How much and what kind of contact should the birthparents and the birth grandparents have in reguard to maintaining their relationship with these children? Has anyone ever had an open adoption and later felt thay had to cut off or limit the contact ?