that's far less complicated than most adoption stories! so dad was in a relationship, it broke down, mum left. she did so because of probelems in her life, she could barely look after herself, let alone a baby. she was ill in someway ( depression, drugs, alcohol addiction)
she didn't leave BECAUSE of the baby - he/she was very small and didnt do anything wrong. most mothers don't leave their children so its hard to understand why she did, but she was very mixed up in her head
then dad got in a relationship with mum, she grew to love the baby and became his / her mummy.
there are different parts of being a mum - conceiving, carrying and giving birth to the baby, caring for it and the legal bit of having parental responsibilty
for some children, they have the same mum for all 3
in many families, its different ( child will knwo others children in class with step parents)
parenst will need to show child photos of birth mum and explain more about why she left and why the child cant see her again right now
if she is still alive you shoudl be aware that child may try to contact her him/herself, so parents shoudl offer to help with this is at all possible
saying " you cant do til you are 18 " is just stupid - if child want to he/she will just use in internet / FB etc
given that other adults knwo the truth here, you really need to tell child asap, befroe it coems out another way
HTH