I am very suspicious of this as well, or something very similar... I just don't think dudes randomly start conversations with random friends of their gf's on facebook and tell them how they are dating their friend. I don't see why he would start talking to your sister, unless he knew exactly who your sister is...
I bet you anything your gf was living a double life, she probably had a 'casual' (sexual) relationship with this dude on the side while being with you for 1.5 years... The fact that this guy started messaging your sister just a few weeks after you met her parents and stated to bring up marriage is not a coincidence.
It could be that she told him, "We have to stop doing this because my bf (you OP) met my parents this week and things are getting really serious, I think he's serious about marrying me..."
The other dude pretended to be cool about it, probably told her "okay" but got vindictive on her and just like GifGafls said: wanted to fuck up your relationship, basically bringing it all down with him, since your gf now kicked him out.
This could be a very likely scenario OP... I am just not convinced that the other guy randomly messaging your sister out of the blue is as innocent and as random as it seems....
This, of course does NOT get your conniving, cheating, dirty Girlfriend off the hook, but I would say tread carefully and do some investigating before goin all out and breaking up with her.