The other girls have given you the same advice I would have - answer the direct questions to the best of your ability and trust that she will form her own conclusions, and that she will be okay. Sometimes we are exposed to sticky situations and our opinions and understanding grows and changes as we get older - for now, knowing that the baby's mum lives with her mum and dad and they will be helping her with the baby is probably going to be enough to satisfy her for the time being.
Please just know that none of us are judging you regarding this situation - it is NOT your fault that your ex has chosen this. People do strange things sometimes and without knowing anything about the girl or her situation, or your DDs dad and his situation, I won't make assumptions about the relationship - it's not my place. I just wanted to wish you and your DD all the best, it sucks that her education about blended families has to be this way instead of a gentler way, but I know you will all get through it okay. Hugs.