Adopting a roomate's baby?

[Confused]

So a girl called 'Margo' moved in to live with a couple called 'Sinead' and 'Alfred'. Margo and Alfred got a little too friendly, long story short Margo's pregnant. I now want Sinead to be the mum, so I want her to adopt the baby. When I go to the computer to adopt the baby (Called Axel) is not there to adopt. Is there a way for Sinead and Alfred to adopt the baby? So they can be the parents and Margo can not? I haven't found anyone else ask about this before, so I doubt that Sinead could. But I really want it to happen. Any suggestions, or is this not a thing?

[Playalot]

Hi Confused and welcome to the forum. What you want to do can't actually be done in-game but you possibly could achieve this by using cheats and editing everyone's relationships in CAS. This can be a little glitchy so just be careful and make a back-up copy of your game just in case.
Open the cheats console and type in 'testingcheats on' <return> then type in 'cas.fulleditmode on' <return>
Next take a family member to a dresser or shift click them and take them into CAS to edit them.
Then just edit the relationships between them and return to live mode and turn off cheats.
Carl's written a guide on cheats which might be helpful for you.