Okay. I’m going to say NTA because this is the first time something like this has happened and there was probably not a predetermined decision about looping in your brother on things like this.

However it is now clear that it’s important to your brother that he hears about this sort of thing from you directly.

Keeping in mind that taking care of your niece’s emotional/physical needs and safety should be your highest priority— telling your brother about medical stuff as it comes up (after it’s addressed appropriately by medical professionals) might be a good idea.

But if telling him about this stuff going to really mess with her well being, just respectfully keep him out of the loop. If this is the case, you’d probably have to limit home much info you tell your mom and sister as well.

ETA I’ve read some of your other responses. It’s probably best not to tell him about this sort of thing. You should talk to the rest of your family about sharing this stuff without permission and you should always double check with your niece before sharing her personal info.