Is guest right a thing any where in history? I understand the historical precedence for condemning kin slaying, but is breaking guest right as terrible in real world history or is that a unique part of ASOIAF?

Absolutely. 

In many premodern cultures, guest right was considered sacred and breaking it was right up there with matricide or patricide in terms of crimes against the natural order set down by the gods. In Greco-Roman culture, for example, the right of xenia or hospitium was enforced by Zeus/Jupiter in his role as Zeus Xenios. Zeus would enforce this by disguising himself as a beggar (sometimes Hermes/Mercury would hang out with him, as he was the patron god of travelers) and then showing up to people’s houses, punishing people who turned him away and rewarding the generous. 

Ovid formalized this tale in his Metamorphoses, where he tells the story of  Baucis and Philemon, who take in Jupiter and Mercury and treat them generously while their rich neighbors bar their doors. In return for their generosity, Jupiter and Mercury spare them from a flood that wipes out the entire town for failing their duty of xenia. 

Likewise, a lot of Greek tragedies have their roots in breaches of hospitality. The fall of the House of Atreus is littered with murdered guests and guests being unwittingly offered human flesh, the Acheans during the Trojan War have the support of Zeus because Paris abducted Helen while a guest under Menelaus’ roof, Penelope’s suitors in the Oddessy die because they have abused the right of hospitality. 

And you see this in other cultures too – in Norse mythology, Odin/Woden frequently disguises himself as a traveller and shows up at the front door of King Geirodd to test him. Geirodd, being a sadistic bastard, has his guest chained between the two fires of his hearth to torture him and refuses him food or drink. Geirodd’s son takes pity on the traveler and gives him food and drink. Odin reveals himself, slays the wicked king, and elevates the son in his place. 

(btw, Tolkein totally stole his look for Gandalf)

You can find very similar stories in Celtic myths, and in the Upanishads of Hinduism, where the maxim “Atithi Devo Bhava” means “the guest is god.” 

Guest-right is not GRRM’s invention.