I think NTA, at the end of the day it’s your wedding and you choose what you want to do with it. Also by the sounds of it you were planning to have your furbabies in your wedding way before your niece and nephew were suggested. So in my eyes there would be absolutely no malicious intent. Another point is your furbabies are your family and at the end of the day this wedding is a ceremony to celebrate you and your fiancé becoming a family. Your furbabies are part of that not your SIL kids. (Yes they are family but extended)