This has been going for months? Huh.
If you follow my work, you know I really hate the phrase “Nipping it in the bud.” But in this case? It applies. The problem is you have been giving your niece mixed messages about her treatment of you, so you need to get a message and stick to it.
Whether it be that you are fat or your eyes are brown or your hair is curly or your legs are short, we need to help children understand every human is born different and those differences should be seen and appreciated. Not teased and not poked at.
Five-year-olds are known for their noticing of differences; this is an age where you can see a little bit of tribalism among the kids. You will start to see little cliques forming, and this is the human desire to feel safe among what looks like you. You see adults who are still the same way.
If we allow children to form and stay in these cliques, they can easily become mean and even cruel.
So, take your niece to the library and take out some books about how people look different. From disabilities seen (wheelchairs) and unseen (deafness and brain issues), you can begin a conversation with your niece that promotes empathy and compassion.
I know our culture hates the word shame and runs screaming from it, the ability to feel some shame means you have a working moral compass. I do not want you to shame your niece, but I want you to call a thing a thing. “When you poke me and laugh at me, you are hurting me and it is not okay. I love my body, and I want you to stop. Now.”
Do not shy away from it, do not insult her, do not become irate. Stay matter-of-fact and repeat as necessary.
Have you spoken to her parents about this? They can also support you, not by shaming her, but by promoting how people are different, and we do not laugh at these differences.
Finally, she is not trying to be awful. She is not bullying you. But you are not doing her any favors by letting her off the hook here.