What should I do?

Sorry it's long. Hope it makes sense.

July 2014 - my brother in law and sister opened up a business (bought out his old boss), we all helped to make it all work, advertising, painting, building furniture etc.
Brother in law told my partner to go do his course, & when he had completed it they would put him on in the business as a therapist, off my partner went.
In the meantime I helped out as receptionist for 2-3 days a wk when my sister couldn't be there because of uni, which again no problem, they promised me the fulltime position for the new year.
Once my partner had finished his course my brother in law turned around and said they've decided not to go with his field of work and continue with what they were doing.. annoying but fine.

2015 came around and tho seeing my sister often and asking she never said anything about me working.. 3 days before she was due to start uni & me work, she turned around and said she had given the job to a friend instead.
This pissed me off no end.. she was always a friend over family person, but knew that I hadn't applied for Christmas jobs because she had 100% told me I had this job.
There was a big argument between us all & we haven't been that close since.
Since then she favours my nephews over my kids, takes them out, visits them not us even tho we are just around the corner.

I've tried hard to try let it go.. things have just started going good again between us.
So 2months ago my partner started up his business and we are slaving away trying our hardest to get it up and running with no help (me 30wks pregnant with 2 kids already), my sister tells me that (after hiring a therapist in my partners field) they are going to again steer away from that because they aren't making much money and go with what they originally were doing.
I asked her if she would mind putting my partners cards etc in the clinic and referring people to him.. the answer "sure no worries".
Never happened.

Brother in law told my partner the other week that he needs to go and just "get a job because nothing will work for him" (which pissed me off again) because we are funding everything ourselves when they were given all the money to do everything.
Only to find out they have now hired 2 guys to do the same job as my partner in there clinic, because they don't want my partner getting business.

I'm sooo bloody frustrated and angry!!
I feel like they just don't give a f&#% and just expect everyone to keep helping them & using everyone as a door mat to get whatever it is they want. ?

I honestly feel like punching them in the face everytime they come somewhere because they act like they've never done wrong by anyone.

Should I just cut ties?