Can You Go On A Break From Your Own Sister?

My girlfriend and I are going to get married at some point, and I don’t want my sister anywhere near the wedding. As a matter of fact, I don’t want her to be a part of my life at all. She has a toxic and abusive personality that has been driving our family apart for years. She cannot be reasoned with and she refuses to seek treatment for her mental issues. I pity her, but I want happiness in my life. As she is now, she has no place in that future. She might improve, but here’s the thing. She has no job, no degree, no driver’s license, no friends or social life, no self esteem, and no motivation to do anything besides watch Friends on Netflix all day long. She can’t survive on her own and my mom will never cut her out the way I aim to. And unless my sister improves, which sadly I do not see happening, she will always be leeching off of at least my mom. 

So, what would be the best way to cut her out without also separating myself from my mom?

—-I Won’t Be There For You

How YOU doin’, IWBTfY?
So, like, your sister has issues. This much is clear. If she were a Friends character, she’d be Ursula, or probably a clothed and possibly reasonably good-looking version of Ugly Naked Guy. I won’t speak to her divisiveness or mental struggles, but I will say that, just although there’s nothing outright wrong with Rachel’s boyfriend Paolo popping a woody under Phoebe’s massage blanket, the problem arises (get it?) when that relatively predictable physiological response is directed at Phoebe as a come-on. In this vein, there’s also nothing inherently wrong with sitting on the couch all day watching TV, a.k.a. “being a slacker.”
In my day, slacking was considered, if not phat or sweet or tight or even chronic, definitely aight, cf: Troy Dyer in Reality Bites, who despite being the smartest guy in the gang’s friend group, spends all of his time cracking wise about Good Times reruns and taking bong rips on Winona Ryder’s couch. For as long as there have been couches and television, there have been grown up men- and women-children slacking on them, and far be it from me to intervene in this artform.
However. You’ll notice that Troy Dyer’s bong rips and wisecracking take place on Winona Ryder’s couch and not his mom’s. That is the primary difference between your generation and mine. Your generation’s parents (aka the only slightly older members of my own generation), having been unceremoniously kicked out and cut off by our own parents in the ’80s and ’90s after an unimpressive ’70s childhood spent being largely ignored, did what we all do, which is parent against the parenting we received.
Add to this the unfortunate economic realities that have crystallized since the ’90s — wages remain similar but the rent on, say, Monica and Rachel’s ludicrously huge West Village apartment has risen about a jabillionfold, from an untenable “rent-controlled,” let’s say $1600-2000 per month in 1994, to no amount of money in 2018 because that building has been converted into $40 million condos owned by some Russian nickel magnate’s granddaughter and she never even goes there.
If you are already predisposed to magical thinking (as anyone who watches Friends is) and your foregrounded assumption is that your job, even if you get one, will be a joke, and you will always be broke, and your love life will always be D.O.A., and on top of that you have multiple issues and an enabling parent, then unfortunately you will always be stuck in second gear and it will, indeed, never be your day, month, or even year.
And now to the matter at hand: You are, in fact, not there for your sister when the rain starts to pour, because her issues are harshing your mellow. This is 100 percent okay; you can have boundaries with family members if they’re always wigging out and you’re always bummed out at them. However. You can’t do anything about other family members’ continuing embrace of those people, and this is especially true if you’re gearing up to have a wedding, which, I hate to tell you, is an event that straight-up sucks if you’re on the planning end of it no matter what you do. 
That’s actually why weddings on ’90s sitcoms only had the primary members of the cast in attendance — like, I’m pretty sure there was a wedding on Wings at some point and the only people there were Joe, Brian, Helen, Roy, Antonio, Fay and Lowell. This was not, as you might suspect, because the producers of Wings didn’t want to go through the trouble of casting an extras or day players for one stupid subplot, but actually because ’90s sitcoms served as an excellent model of how at the milestone events of your life, you should only surround yourself with the people you actually like, or who, at the very least, will offer an excellent comedic foil to your existence.
You’re going to have to make a decision. Either you have a big wedding and invite everyone and just steer clear of your issue-laden sibling and assign one of your groomsmen the 24/7 duty of running interference and embrace the suckage (the most accurate translation of Nietzsche’s amor fati, in case anyone is wondering) — or you elope, and invite neither your mother nor your sister, and save $20,000 that you could put toward one and a half month’s rent in New York City. After all, that’s definitely what Chandler would do (or what he would have done if his wedding to Monica had any input from him), and I have a distinct feeling that you’re the Chandler in this particular milieu.