apushatayid,
Well, perhaps the poskim DO make that comparison.
The chibuk venishuk thing is by a sister, with whom technically the regular issur should not apply. There is no hirhur issue with a sister. And still, the poskim say that any affectionate touch falls into that prohibition.
But by a niece, where there is hihur, she is like any strange girl. Halacha does not differentiate between her and any other girl.
This is serious stuff. Speak to a reliable Rov. Don’t pasken from a coffee room!