SUBREDDIT: r/relationships
TITLE: Me [22 M] with my mom [54F], I've boldly notice favoritism and want to cut contact, how?
POST: Basically the last couple years my mom has pressured me to let her claim me on her taxes as a dependent in exchange for $120/month (1/3-1/4 of my living expenses). I was ok with this, but i recently learned that she has also been receiving $3,000 extra per year because of what i've been paying myself in tuition. 

I brought it up this year when i heard about it and she refused to reimburse me the $3k for this year (or any previous years), so i had to file independent and only receive half of that.

I recently found out that she has been giving my sister [23f] "kickbacks" every year, and not me, which she denied. 

I've been a little sour about the tax thing, because affording tuition has been difficult and that extra $9k would have been amazingly helpful. I realize that money is now gone and I should have filed independent if I was, and i have no one to blame but myself.

I just found out that my mom is taking my sister on a cruise (she has also given her a month long vacation in Europe in the past), in a couple months and neither one of them have told me. This was set in stone before the tax thing even came up. 

Now I don't think I'm entitled to a cruise, but this favoritism has made me want to completely cut contact for a couple months at least, and I'm just looking for some terminology to best explain it to them. My mom is incredibly good at interrupting, so a phone call would have to be well-done to work.
TL;DR: 
mom buys her golden ginger child a lot and tries to get money from me, how to tell her I'm out?