It could be the rambam thought it was gross to hug and kiss an erva. Who says he thought it was gross to kiss a 6 year old niece?
Note, the rambam doesnt say it is gross to kiss a sister, just an adult one. Maybe he things its gross to kiss an adult you cant marry. Maybe its ok to kiss a child(also maybe its a different kind of kissing than an adult)
If the Rambam goes to the extent of saying that Hammechabbek 'achat min ha'arayot she'ein libbo shella'adam nokefo 'aleihen, 'o shennishak 'achat --mehen-kegon 'achoto haggedolah, va'achot 'immo, vechaiyotze --bahen-'af 'al pi she'ein sham ta'avah velo hana'ah kelal, harei zeh megunneh beyoter. vedavar zeh 'asur hu, uma'aseh tippeshim --hu-she'ein kerevin le'ervah kelal, bein gedolah bein ketannah chutz meha'em livnah, veha'av levitto, how much more would it be distasteful and assur to have negiah with a niece. (Issurei Biah 21:6)