When it comes to forbidden relationships the rambam makes a distinction between certain family members when it comes to hugging them. For an aunt the Rambam writes that it is foolish but for other family members he writes that is it is forbidden to hug. Why does he distinguish between arayos if they are all forbidden?
Although all arayos are forbidden, there are certain arayos to which a person is not naturally attracted (the yetzer hara does not incite to sin) — such as a sister — and which therefore do not carry the same stringency as other arayos with regard to hugging and kissing (Rambam, Issurei Biah 21:7).
However, with the exception of a mother and daughter, and grandmother/granddaughter (according to many authorities), and a young sister (up to the age of 11), most authorities make no distinctions, and prohibit the hugging and kissing of all arayos.
Sources: See Iggros Moshe, vol. 2, Yoreh De’ah 137 (concerning the prohibition of kissing and hugging cousins), 144 (concerning sisters); Chochmas Adam 135:1 (concerning relatives), and 134. See, at length, Nit’ei Gavriel, Laws of Yichud, pp. 339ff, who quotes many details of these laws.