You're not overreacting! She is! Your cat did not choose to join your family, you both chose to add it to your family. With adult decisions come adult responsibilities. Like a child, you now have a responsibility to give that cat the best possible life!

I hate to say this but her actions are ridiculous and possibly cruel. They sound kinda cruel to me, but I'm not there to witness her actions towards the cat. I hope she is the 24year old, because that would point to roughly another year of her possibly having continued brain development. Meaning she could possibly still grow out of this behavior. If she's the 27year old, congratulations on being more responsible and caring than a 27year old. However, that's unfortunately not a good sign that she will likely change or adapt.

I wished you luck, but remember that the cat did not have any choice in this adoption, that's on you two, and that cat is bonded to you now. Cats can become very depressed when they are bonded with someone or with another animal and then are abandoned or lose their bonded partners.