The question was Does the mishne berura say that you may not touch your niece?
That the halacha is the same for any girl, related or not, may be true. Though the identical halacha may be that it is muttar. So does the MB say its Assur?
Siman 21 in EH discusses various things not to do with arayos, but niece or neighbors daughter are not arayos.
FYI if you touch your niece or any other girl, you may end up in jail, but that is a different discussion