Adopting a child already in the household

@nannarosen113Are they married now or just living together. Does the relationship status between your Sim and the child change if you get married?

The MCCC mod allows you to edit the relationship status between two Sims, including father, mother, etc. Make a fall-back save, then give it a try. If you like it then keep going, if not then remove the mod and go back to your fall-back save. I made a whole NPC family for my current Sim so she now has a mother, father, and younger sister. There are probably other mods out there that can also do the trick but I haven't used anything other than MCCC. Make sure to use the usual precautions when making changes to your mods folder.

For doing it without mods there is a testingcheat called

relationship.add_bit {yoursimID} {targetsimID} {relationshipBit}

It adds a relationship bit (status) between two sims where {relationshipBit} can be one of the following values:
romantic-Married
family_husband_wife
family_grandchild
family_son_daughter
family_brother_sister
family_grandparent
family_parent