AITA for not giving a cupcake to a child who is not originally on the guest list?

I (24f) recently returned to my hometown after being away for college and work. My grandma often hosts potluck party where her 3 kids and their families can hang out. I am a pretty good baker and the couple of times I brought baked goods over, I got lots of compliments especially from the kids. <br><br>For the potluck we had last week, I had promised to make a cupcake for each and every kid (8 kids all under 9). The adult would get a cake instead of a cupcake. I carefully colored and decorated the cupcakes and when I was putting the final touches, my sister (30f) told me if I could also make one for her daughter, E (7f) who was sick that day and missing the party. I told her I couldn't as I had already promised the kids and decided to finish them first before making one for her daughter. <br><br>At that time my sister offered to take her daughter to our grandma's house but I said I didnt want to and that she could join the party if she wanted. She didn't so I ended up going without her. After the party, I got lots of thanks from the kids who had their cupcake and I was gifting one to two of the kids who didn't get to have one because they were not there (my nephews). <br><br>My sister called asking if I could give E her cupcake I had made for her and I told her I didnt as it was not good to promise one child an item only for themselves to get it just because they are her daughter. She emailed me saying that I was a jerk who does not like kids and a AH for not giving her daughter a cupcake when I was willing to give my male nephews who were not even grandpa's grandkids a cupcake. <br><br>ETA: I have 2 nephews and 6 nieces. The 6 nieces are my sister's daughters and my brother's daughters. The party cupcake were given to my 2 nephews because they were not able to attend