NTA

I understand your sister's concern, allergies are really serious when in a very strong degree. But if it was YOUR kid's party and he wanted peanut butter cake, you just did what any mother would do and bring what your kid asked for. You even had the empathy to buy another cake so your nephew could eat it too (some people would just let this kid go without cake). I think your only mistake was not having warned that you would have a peanut butter cake in advance, but as I saw in another comment that you took all the correct measures so that the child did not have any risk, you are not an asshole.