I made a child and young adult sim a few days ago, they are siblings. Now I made a adult woman, but the game doesn't allows me to set her as the siblings' mother, just sister or roommate.

Save the adult woman into your Library. Then bring young adult and child into cas. While there, open your Library, merge adult woman with family, then set the relationships. I do this trick all the time, for things like adding the famous Cornelia and Gunther as Mortimer's parents, then killing them so the famous Goth ghost are in the community graveyard and properly related to the Goth family with all correct family tree connections.

Merge directly from your Library, not from anywhere else, otherwise it wont accept making them related.

As a final note, the game can get finickity if the age gaps are too close. Like making a teen the child of a young adult. It might reject. So, if it is being a jobsworth about this, make the adult into an elder. Then, after the game accepts the new family relationships in cas, and after you place family into a home, go back into cas and make the elder an adult again, the game accepts it if you do it this way. (Maxis did this same trick for the Caliente girls. Note how the adult is the mother of two young adults. You could never do this without using the trickery I've just explained.)

Hope this helps you get what you want.

Sorry for the necro. This definitely works, however there's one problem, it completely wipes out the added sims relationships. I assume this is because it generates a new sim id. This just wont do for me cause my sim has way too many family members and friends to have to go and get them all back. I don't suppose there's a workaround for this problem is there?