Interesting, almost 6 years with my ex gf and after silent treatment --> informal breakup --> 1 week back together ? --> silent treatment for a week --> moved in with a co-worker she meet on her new job (I'm talking someone she meet for less than 3 weeks) and cut ties with her mother and sister and told me she was going to marry this person

does not compute.
I don't know how thats works

i mean they barely knew each other, only contact they had was at work and maybe over the phone, who knows what she told this person about me and honestly you gotta be one desperate individual to not see there is something wrong.
Regarding her close family and to some extend her friends, don't worry about it mate they have all the blame being enablers in her life, this people don't accept to be told they are wrong so they just agree with whatever she claims.
My advise ? Joke about the whole situation.