How to add Agnes Crumplebottom to the family tree?

It is, but you'll need to use the cheat that allows you to completely edit your sims in CAS. By doing this in World edit mode, meaning not actually playing with the sims, it should preserve what the game has already set for them. To play it safe, before doing this make a separate save of your game. This was if something goes wonky, then you won't lose what progress you currently have.

"Sthenastia;d-999619" wrote:
I'm working on my new save file with an improved lore of the orginal sims but I have a problem. I've added Mortimer's parents from The Sims Unleasched and due to fact that I have Cottage Living I would like to add Agnes to the Cornelia's and Mortimer's family tree but I don't want to break Cottage Living mechanic. Is it possible to do that without any mods?

Click on Agnes to "modify in CAS". In CAS you will save her (household) to your album. Then leave CAS without saving. Later you can do whatever you like with the album copy without affecting the real Agnes. You'll get her picture and name etc, and unless the game-Agnes builds a new family there would not be any relatives anyway. You would need to add her from Mortimer's household, if he is long gone I have no idea how...

Honestly, the easiest way to do it without messing things up is with MCCC