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

I feel you. I was in the same boat :grimace:
There's a cheat for it, but it's very long winded.

First you need to turn testingcheats on, then type in Sims.get_sim_id_by_name First Sim for the first sim. You're going to get a huge number/letter combination that looks like gibberish. Type that cheat again for sim number two, and see another long number/letter combo.
Those are their IDs.

Now, (here comes the annoying part), type in relationship.add_bit FirstSimID SecondSimID family_parent. Don't interchange the IDs. It'll look like two huge number things next to each other, separated by a space, with "family_parent" after.
This sets "First Sim" as the parent of "Second Sim."

Now, you're going to have to do that cheat again, except it'll look like relationship.add_but SecondSimID FirstSimID family_son_daughter.
This sets "Second Sim" as the child of "First Sim."

The game should register those two sims as relatives.
It's the only way :(