Rav Moshe, in Igros Moshe Y.D. 2:137, seems to say that a niece is prohibited by Torah law. Rav Moshe writes that a brothers/sisters daughter (a niece) is quote, issur gamur. Only then does he move on to discuss the status of a sister, fathers sister, and mother sisters - and concludes that in those cases it is a rabbinic prohibition.

Shalom, Thank you for writing to us. I re-read the responsa in question. While what you write does seem to be the understanding from the first paragraph of the response – that does not seem to be the conclusion. In the last paragraph Rav Feinstein zt”l writes “… but with one’s sister, [one’s] mother’s brother, [and one’s] father’s brother, where there is an assumption that there is no chibah (affection), he would agree that it is only a Rabbinic prohibition”. Based on this we need to understand the words in the first paragraph that kissing one’s niece is a “complete issur” (issur gamor) is not coming to say a Torah level sin, but rather a complete Rabbinic sin, as opposed to Rambam’s words about kissing a sibling, that it is (only) “disgraceful” (meguneh). I hope this is of some help. Blessings.