Awwww! This post is sweet and it must be very hard to deal with this when you love your brother.
Unfortunately, for many young people (myself included at that age!!!) A romantic relationship becomes way too important. Perhaps (I only said PERHAPS) because we're so sexually repressed and private in this country that all kids want to do as soon as they possibly can is get hopelessly emotionally attached to someone they're having sex with.
I used to live in Europe where people are much more frank and open about sex from much earlier ages, and they're so much less messed up later on about sexual relationships, and they have less pregnancies and all that...It's like, people want to take their first step of "independence" here by having a regular sexual partner and playing house because it's so hard to wait for the perfect soul mate to show up "to marry" and it's so hard to pay bills, have friends, go to school, work...there's all this financial trouble, and the "significant other" seems like the best thing going on at the time.
Granted, if all the girl did was retreat into your brother's room and she's not giving, helpful and considerate when people are extending her a favor like a place to live, then, she's probably not a fantastic person. For a while I was living in a guest house behind the parent's house of my no good ex boyfriend from a million years ago. I worked like a slave to fix up that guest house AND help them with their own house work, pets, shopping AND I PAID RENT and worked....cuz I was dumb and thought my "adult relationship" was really great.
So anyway, back to your brother. Aside from the fact that she may not be a regal catch. All the other stuff is his choice too. Like being with her, eating junk food, adopting cats, etc... If he wants to manage a Costco, and he works at a Costco, then he's on the right track there at least.
Moving in with a girl and adopting cats is NOT a huge life commitment. I don't mean to sound like a cheap person with no morals, but everyone does it. At least tons and tons of people do it. I did it and I came from the MOST CHRISTIAN of proper households. All my friends did it. Live-in boyfriends and girlfriends come and go and cats get batted around like toys. I can't count how many times friends and friends of friends were trying to negotiate where to place cats when people broke up. It's like a classic cliche joke: What to do with cats after break-ups.
Ideally, he'd have higher standards right out of the gate for school and career. But you guys can't make him. At 21 though, your parents would be fine to stop paying for his school. He can always go back one day when/if he is more serious about life. Not everyone has solid goals in their early 20's.
You're right, a pregnancy could happen any minute, and then your brother will be REALLY tied down, but he's an adult and he'll have to navigate that. He'll work hard enough to support his kids if he's a good guy. They may even help him mature. Lots of people have kids young and make it work even if their relationships dissolve later.
I dumped several bad guys in my 20's and worked and saved to move to NYC and eventually started my own company, got married, now I'm a divorced mom of three in rural PA with a great life and lots of projects going on....my brother drove my parents crazy by never getting "serious" and he never married and still teaches kiteboarding in Florida. He just likes to party. And both of us were raised with very strong work ethics etc. being told we should "Not have sex until marriage, go to college, NEVER live with someone, get a good job, blah blah" You just never know what people will do.
Anyway, your brother has to learn his own lessons and live his own life. You guys have to let him. Any interference or support at this point only enables resentment and dependency. He's too old to have people monitoring his weight and choices. Let him go! Love him unconditionally. He'll come around. Or work things out somehow.