Ah first times, I think any first time you do between the ages of 15 - 18 are probably the best. Invigorating and testing the world around you. As I got older I chases many a first times in order to get that rush again and never was able to.
That was until becoming a dad for the first time, I can't explain it but it is same as you and your neice. It is your kin. The reason why you live to protect and to teach. Funnily enough I would have agreed with you in regards to first times not being a first time. But you will experience another first time when your brother has another child. It won't be Little steps but it will be the first time experiencing a second. I can't explain it but having another kid is just as powerful as the first time. I've never experienced anything like it.
Great weekend topic, it brought a smile to my face.