How to tell birth-siblings of adoptions

Hi all - I'm the amom to a dd, 9 month old. DD has been with us since she was 3 months old. The bmom hasn't explained the adoption to her 6 year old daughter and doesn't know where to begin. She has asked us to be present and help her when she tells her 6 year old daughter about the adoption. Her 6 year old has been asking about her sister and bmom just tells her that she is living with us and we are taking care of her. She also has an 18 month old son that lives with his dad - so this isn't unusual to the 6 year old. So all of this coupled with the fact that we are moving out of state, doesn't seem to lead to a positive conversation that day. My question is this - how do we assist with this conversation to ensure the 6 year old doesn't come up with fears. My fear is this - that the bmom is using this situation to make us look like the 'bad people' - that we took her sister away - so she won't have to tell her it was her decision. I don't mean to offend any of the bmom's out there -and understand I'm not basing these fears on generalizations - they are based on past experiences with our bmom. Any suggestions or reference materials that we could use would be very welcome. Our main concern is for the 6 year old - her understanding (at her level) and comfort with the situation.