AITJ for not giving my sister her son?

My sister just had a baby a few months ago. From day one, she basically ditched him on me because sher got a new boyfriend. She barely knew how to take care of a newborn, and I ended up doing all the thing like midnight feedings, diaper changes, doctor visits the whole deal. I was able to do that coz i work from home.

Fast forward, her boyfriend is gone. Now she suddenly wants her kid back like nothing happened. She’s even talking about asking the father for child support but from the way she’s talking, it sounds like she just wants money to cover her partying, shopping, and basically her vices.

And here’s the part that hurt me she straight up said I want the child because I “can barely get mine” since I have PCOS and in my mid 30's. Like… seriously? First off, the baby isn’t mine. Second, I’ve been the one caring for him while she was out living her life, and now she’s trying to gaslight me into giving him up because of my fertility struggles?

I love my nephew, but I’m exhausted and honestly feel like she’s just trying to offload respobnsibility onto me and the dad. AITJ for saying no and not giving her the baby just because she finally decided she wants him?