Yíiyáh is a Navajo expression that ranges in meaning from ‘scary’ to ‘dangerous’, and is often said in jest as part of the (very many) ways of teasing relatives or members of other clans.
Most of the time, it has the slight connotation that something is “going to come after you” if you continue to meddle. In the broader sense, it’s the idea that something unpleasant will come of a behavior.
“Yíiyáh, she’s your cousin.” Is an example of something one might say to another that acts romantically towards someone who shares a subset of clans - even if they are not directly related. In this sense, it indicates more a taboo.
In all of these cases, it’s a less formal way of saying “You should stop”.