A strong, mutually-consenting, relationship between a male and female non-biological step-siblings that is legal within the jurisdiction they reside. It may or may not have a sexual element.
The term familiance does not apply in cases where it is legally described as incest.. It doesn't apply when the relationship is more aptly described as child abuse.
Entomology: Framilian is a derived word that orginated with the word bromance. Bromance is a portmanteau of the words bro or brother and romance. Editor Dave Carnie coined the term in the skateboard magazine Big Brother in the 1990s to refer specifically to the sort of relationships that develop between skaters who spent a great deal of time together. The word was intended for only platonic friendships so a second word framance was coined to described a similar relationship with a sexual element.
Female equivalent words that were coined were sismance, for a platonic relationship, and soromance for one with a sexual element.
Same sex step-sibling relationships with a sexual element, where it is legal, and where one or both is really straight are better described as a framance or soromance. Same sex step-sibling relationships, where it is legal, and both are gay or lesbian are better described using the terms gay-romance or lesbian-romance.