Goodness Ash - not sure how I would handle that as my 5yo is a parrot and it is a complex situation to explain without seeming judgemental. I would focus on the direct questions she is asking and try not to elaborate too much if possible. So if she is asking why the baby is not in her Dad's house then perhaps a simple statement like 'Girls name still lives with her mum and Dad and the baby will stay with her for now'. As to the being an adult, tricky one. Is saying that people choose different ages to have a baby OK? We have had this discussion as my nephew is 19 and his mum was at school with me. I just said that every family is made up differently and people do things in different orders, we have a very interesting family with steps/people deceased so have had some curly questions too.
Simple statements of fact and checking in with her as to if that makes sense and answers her questions will help. Remember if you are ok about it all in terms of explanations she will be too.