AITAH for refusing to give my sister her daughter?

I (28F) have an older sister (40F) who gave birth to her only daughter 10 years ago. At first, she was hesitant as she was interacting with her newborn because she was afraid of neglecting her son (4M at the time), but she grew close to her daughter very quickly — literally in a matter of days. Since my older sister was always visiting my parents, I used to carry her daughter and make her sleep a lot of the time. There were days where I visited her house specifically to go on babysitting duty while she had an errand to run. I grew very attached to her daughter. She was my first experience with a baby as I don't remember my younger sisters as babies, and I was incredibly surprised with my capabilites. I fell in lover with her and kept imagining having my own daughter, even was I was still too young for marriage and children and I wanted to fully dedicate my time to my education and career before considering a family.

When my sister's daughter became a month and a half old, my older sister woke up one morning to find her daughter's eye extremely swollen and releasing foul-smelling pus or discharge. Eventually, she had a doctor's appointment and the doctor informed her that her daughter has an extreme infection and may go blind in her right eye unless she undergoes surgery. My sister and her husband couldn't afford the surgery. My sister couldn't handle the truth, so she gave her daughter up for adoption because she couldn't carry the responsibility, and visited my parents a few days later in tears and she tells us what happened. We all called her crazy, especially my mom who absolutely lashed out on her. I told my sister to bring her daughter back and I'll take care of her until she's ready for the responsibility, after asking for my dad to carry the financial responsibility (since he was heavy on debts and I needed his permission). He agreed right away and him and my mom tried checking with me a million times if I was ready to carry the responsibility since they're growing old and can't do more than play with the baby and I've nearly finished first year of med school. I was 18 at the time and they were heavily concerned over me sacrificing my youth for my older sister's baby who should be HER responsibility. Eventually, we all agreed in me taking in her daughter. Years have passed and the whole journey over the past ten years was really hard. The hardest part wasn't the crying or sleepless nights or the nights I was close to breaking down because I didn't know what she wanted. Raising her was the hardest challenge and I never expected to have admitted that — I truly then understood how strong my mom is, although she called me stronger for choosing to become a mother at a very young age. Mind you, I did not officially adopt my sister's daughter. She's still considered her daughter. Now, the beautiful girl is 10 years old. She really is blind in her right eye but her personality and character is so lovable. She's smart, beautiful, kind, helpful, and everything I could've asked for in my own daughter. I'm 28 years old now — still unmarried but I spend my time focused in taking care of my family and education.

After my older sister saw her daughter thriving, she sat me down and told me she wants to take her daughter back. She told me she's "ready" to carry the responsibility again and she is very grateful for everything I did the past years. I know I'm supposed to give her her daughter back, but I can't. Why is she expecting me to give up all my hard work just so she can have her daughter ready after all the difficult and ugly parts passed? I'm also sure she wants her daughter back because her son has become troublesome and a little unlikable. I feel guilty for refusing and starting fights over this, but I can't let the girl go.