I want to make a Sim a father of other existing Sims in my household but I can't

you should be able to do this without a cheat. Have you tried it via 'manage household'?
It's an option when you click on their house before you load it.

I should mention too that while editing a family this way I was able to make the woman a mother to her sons but the boys weren't brothers until I played that family. So maybe you just need to play and see if the relationships are sorted.