Dude, dude, dude. This is so messed up, I don't even know...

It pains me to say, but you will need to fight fire with fire and start becoming Machiavellian in how you approach the inlaws.

• Get the fuck out of there. Post in your cities reddit (Toronto? My wife and I have a couch which is yours for free, for a week) or go couch surfing or airbnb. Especially with airbnb: look at one of the less touristy locations (suburbs, midtown). You can do it through airbnb officially for a week and if you hit it off with the host, you can extend your stay for just cash withiut paying airbnb fees.

• Your GF needs to break all contacts with her mom and help her dad find a backbone. It seems she and her sisters don't have the clarity of mind for this, but if the relationship with the sisters is good, then the power is on your side. Basically you deny the parents all contact until they start acting reasonable. Eventually they, and especially the father, will cave after a few months. Nobody wants to grow old alone.

If all siblings work together and basically form a cartel, it will be very effective.

Also, divide and conquer. Use her daughterly charm to win over the father. Let her have periodic coffee dates with her dad where she can make clear to the dad that she loves him, and mom too, but that mom needs to start acting like a responsible, caring parent and that it breaks her hart to have to do this. But keep it fun and light. Don't force anything. No ultimatums. He might interpret it as being forced to choose between his wife or his daughters, when the objective is to just have a good relationship with him, so that whenever mom bitches about the daughters, he'll just snub her and say "you're the one being unreasonabke".

Eventually mom will realize that dad is not in her side and will either try to force him to choose (probably leading to a divorce) or she will come to her senses and - in fear of being abandoned by her husband and her children - will try to reconcile.

Let him come to his own conclusions. Ask dad about his work, compliment him, laugh at his jokes and basically be his daughter.

That makes the choice much easier: do I want a subpar relationship with my shopping addicted old hag, or a normal relationship with my loving, beautiful daughters?

Anybody getting married? Only invite dad. Grandchildren? Only invite dad.

I know, this is not how relationships should work. But this is how you rebalance the power in a relationship.

Because the root of the problem is that mom thinks she has all the power in her relationships with dad and the children, when it should be a relationship of equals.