First of all, congratulations to your sister for getting such a good gift from God. Haha, little girls are so cute. I personally prefer girls. I think since your sister has given birth to a child, you can give her clothes, children's clothes, or small blankets, because you know that newborns need to be wrapped in small blankets.
Or you can buy some children's toys. Nowadays, children's intelligence development is relatively early. You can give some educational toys to your niece so that she won't lose at the starting line. Haha.
Then you can also buy some practical things like diapers. These children need to use them every day. Over time, it is also a considerable expense. You can also ask your sister what they need, and then buy them. Many of my friends ask directly in this way, so as not to buy the wrong things.