A close friend of one's husband who is MORE like a sister and LESS like a mistress ???
friend, bromance, sister, mistress, cougstress, sistress
The female equivalent of bromance, the sistress relationship is one of intense emotional attraction subtended by repressed erotic attraction that is neutralized by an acknowledgement that the two ladies are really, hilariously into one another.
"Liz never stops talking about how great Lesley is. That is one committed sistress!"