The original question stated this was said in the name of the mishna berurah and asked “was he correct”. I’m sure the question isn’t “is the MB correct”, rather did this person correctly quote the mishna berurah.
To rephrase the OPs question, is there a MB that states, as asserted by Reb Somebody Shlita, that it is assur to touch ones niece once she is older than 3? If someone is familiar with this MB can they let us know where it may be seen inside?