sticky situation

So my cousin , who also happens to be my best friend just got engaged . She's 21 and has been with her boyfriend for a year and a half . Don't get me wrong , it's not the age or the time that is causing these mixed feelings in me . And I know it's not my choice .... but she wants me to sign the papers as witness . She isn't doing maid of honour or anything but she wants me up there with her as a bridesmaid . Here is why I'm concerned. My cousin has been having some serious self distructice behaviour . She just got an abortion and found out she has an std .. she is very clingy and latching on to her boyfriend . She's also pushing away the family and all the people who love her recently and her fiancé is supporting all of this and not helping her get better in anyway at all . I think she's jumping into marriage just for the sake of stability somewhere ... she's come to me many times about their issues and has almost broken up with him in the past many times . I just don't know what to think . Any opinions ?