Aang would be very happy and supportive. He has always been a friend to both Zuko and Katara, and he would see their union and the upcoming baby as a positive step towards more harmony in the world. Sokka might be a bit overprotective at first, especially towards Katara. But eventually, he would come around and be excited about being an uncle. Toph would probably make some sarcastic but also very endearing comments about the whole situation.
Well, Aang would be overjoyed. He has always believed in the power of love and friendship, and this would be a great example of that for him. He would offer to help in any way he could, whether it's with some light - hearted stories to cheer them up or using his airbending to create a gentle breeze for Katara when she's feeling warm. Sokka would be initially shocked but then would become super protective. He would start making plans to train the baby in combat when it's old enough. Toph would be her usual self, cracking jokes about how Zuko is going to be a nervous wreck during the pregnancy and how Katara is going to boss him around even more.