I look at it this way. If your parent's didn't make sure you had some kind of marketable skill by the time you were 18 (meaning, they just let the school raise you and didn't teach you squat) then they owe you an education; it's part of their responsability. If they didn't, they're pieces of shit for abandoning their kids and they deserve everything that's coming to them.
My parents got divorced, took it out on me and my sister (mainly me because I don't fight a lot) and fauight through me specifically. My father is shit for savings and would rather go into debt to start up a junk business than help me with anything because, apparently, I'm a bad person for not raising myself properly. Infact, I'm such a bad person that he decided to take the money out of my college savings ($700, not much, but it was a quarters worth of classes and books) and put it into stocks where he promptly lost it. What does he get? I won't talk to him and frankly, when he dies that's going to be one less reason to own a shotgun. I really don't give a fuck.
On the other hand my mothers been screwed up for years and she's beginning to realize that. At least she cares.
I look at it this way; your sister doesn't have a clue as to how to manage money if she's in enough debt she can't live elsewhere, and her boyfriend is a bad choice if he can't bring home the bread(especially at 29, shit, you can get a cheap room and get a student loan, go to college for something that's sure to make money and pay it off by 29). Fact is, unless some really horrendous shit just happened (like, both cars broke down, his tools got stolen, and the rent and amount you needed to live increased all within a 2 month timeperiod) and she wasn't smart enough to prepare some kind of savings, moving back in isn't a good choice. If she got into so much debt due to her own stupidity then you need to point this out to your mother.
You need to go to your mother and point out that if she's letting her daughter make more bad decisions and if she lets this process continue it's only going to get worse. If they decide to turn you into bitchboy then you need to make medium-term plans (within a year) to get out of there before it all comes crashing down. Eventually, her boyfriend is going to show himself to be an asshole, your sister is going to get into monumental debt and she's going to lose even keel, and guess who they're going to come to for financial aid and say needs to get into debt to help them out?
You, and if you still need to live there they're going to use that against you. Family doesn't treat family that way.
And tell her that if she wants the bigger room, she's going to have to pay for displacing your stuff. Personally, I'd build a shed out back of some kind instead of using a storage service; it's cheaper (and probably less work).
