I'm going to sit on my neice
Why are you going to sit on her?
I don't think it's wrong to sit on a baby.
That's harmful and dangerous to do to a baby. You shouldn't put her in hark like that she can't handle your weight.
I can't imagine her being hurt.
Don't you think it would hurt you if someone that much larger than you sat on you?  Babies are easy to hurt and you should not risk it.