“It was my only week this year, it has been over a year, and it’ll be another year until I get a week off again.”
If your profession or job only allows you a week off, surely it is of more concern that you’d spend your vacation with someone you call a “pathological liar”. (Please read up on sociopathy and vow not to misuse terms like this again. To be sure, people toss around equally as serious words with abandon and never know what they in fact mean, e.g., “stalker” and “harass” and what not.)
“When she arrived she surprised us with the fact that she is pregnant. She told us she was trying to get pregnant for the last month but two weeks ago said she was giving it a break.”
If this is not a miscommunication, sounds like another ploy 'cause the second sentence makes no sense and is contradictory within itself (in other words, the chick was “pregnant” and had been trying to get pregnant for … two weeks?). 
“The very first night she came in her fiance spent the majority of dinner playing on his phone.”
And this is different from his usual behavior … how?
“… I had majorly miscalculated this visit.”
I’d say it’s more worthwhile to examine whether you routinely delude yourself or set yourself up for disappointment. In other words, do your domestic enemies/mental Nazis routinely have the upper hand?
What is exciting about this “campus” you mention that tourists would want to see? Rhetorical.
“Girlfriend and I are devastated.”
Really? “Devastated”? Why didn’t you acknowledge reality on the first or second day and then do what YOU wanted to do and leave passive-aggressive sister a/k/a “socio” to her own devices?
“Now folks, I’ve had my entire vacation time wasted and ruined by two childish people …”
If it was “ruined”, it was you who did it in that you allowed it and didn’t make a choice not to do what you wanted to do with your time, without the socio sister.
“… who I’d thought had grown up in the last two years I’d met them seeing as their rushing into having a baby.”
Huh?
“We get out of the car as we’ve arrived and I say some pretty bile things, admittedly bile…”
Presume you intended to type “vile”.
“… I don’t want to expand too much on why I think she is a sociopath unless anyone is actually interested.”
Nothing you’ve said indicates your sister is a sociopath. I suggest you have bigger things about which to worry.
“I have decided that I won’t be around her any more. Ever.”
That’s certainly your choice, and I’d examine, among other things, why you didn’t make that choice sooner.
