What to tell Daughter about Bios?

We are in the process of adopting a child from foster care. We currently are not allowed any contact with the bio family, but at the time the TPR occured, they were extremely supportive of us adopting (No contact order was by DHR - not us). The issue is dh does not want to have any contact with bios whatsoever bc they cannot be discrete with information. They are not dangerous in any way, shape, form or fashion & realize that we love K more than our own lives. I am wanting to keep in contact even if it is just pictures & letters. They love her very much but are severely MR & cannot care for her. Anyway, K's bio parents are brother & sister. We do not know whether to tell her always or wait until she is old enough to understand that it is not something that most people accept. The chances of someone else telling her is slim to none as no one knows other than us, her bio family & the cw, lawyers. Also, how do we tell her without sounding judgemental or disgusted? We are not. She is still undergoing genetic testing but thus far, she has no genetic issues. She does have some severe heart conditions that may be related, but it is unknown at this time. We do not yet know how much she will be able to understand, but she is not all that delayed currently & her therapists think it is more related to her health conditions than her parentage & parents' MR status. Christy