False Sibling Title After Sexual Intimacy

My close friends, let's call them Rick and Ann, have had a tumultuous relationship but are now for the most part happily along.
The main source of their problems have been a third party, hmn, let's call her Jenny. Jenny exhibits many personally disorders, but there's so many to list it would take awhile. Jenny is an exgirlfriend of Rick, and has harassed, and psychologically attacked Ann from even before she began dating Rick. So her continued presence in their lives is an issue.

What has been very unsettling to me, is that Jenny and Rick have obviously had intercourse and Jenny is not shy about sharing that far and wide with all the juicy details intact, but now she calls and lists him as her "brother". I do not think this is healthy, nor is it reassuring to Ann. I have been asked for help from her, so I am trying to figure out how to approach with reasonable advice, however aside from opinion I am not schooled in psychology.

I am viewing it as an attempt to maintain a relationship, any kind of personal one, and stay in his life. My husband also agrees that it's bad, so I know I have a male opinion nearby. Jenny has even gone so far as to make sure Ann has seen/found (I'm not sure which) a captioned photo post about "Had Sex, Calls Me Sister, Makes It Kinkier", so obviously Jenny is aware it is not what one would call normal adult behaviors. Ann has taken all precautions she can to up privacy online and express her discontent with Jenny's presence when she has to accept it, but Jenny still manages to interfere with their life and often reminds me of a pushy in-law.

Ann wanted to get married, now she's very unsure until Rick stands up to the misbehavior of Jenny. As Jenny will purposefully do many questionable things, and says hurtful things and generally is allowed to in some way get away with it without facing consequences. Rick often makes excuses such as "it wasn't meant this way", "You're overreacting", or something similar to undermine how it makes Ann feels. Sometimes he just doesn't even believe her. I have warned her heavily on that.

Opinions or advice I can use to help them?