If the person is committed to using the tool effectively, then of course it is easier than the old fashioned way and why people do it. Your stomach is smaller, it reduces feelings of hunger, etc. It makes you sick if you eat too much. It is a lot easier to get through your day on tiny portions because you are not feeling starving all the time.
The dedicated patient will eat their egg by itself or the small amount of tuna or baked chicken or whatever. Have to be really careful not to use the precious small amount of room on carbs, or there is not enough protein going in. It's a lifelong battle to stay nourished when done correctly.
Where it becomes really problematic in the non-disciplined, non-committed person is that if they are not eating to satient hunger, but for emotional comfort (like my sister) THAT need is still there. If it's not addressed and solved, the person can slowly stretch to accept the bigger portions of food that they want.
She started off with 'one more bite because it's yummy' and then ever increasing amounts, slowly until she was right back up to where she was in weight, but now with a surgically altered stomach that isn't designed for it.
I'm not saying no one should ever do it, or that your sister should definitely not do it, I was discussing the fact that you thought it was literally, physically forced to lose weight and it doesn't actually work that way.
And I personally would not push someone who isn't going to be committed to it. I go the opposite way. I try to talk people out of it.
I have two committed friends, one of them chose the sleeve instead of the by-pass, that is a little better to me. The other I said no, that is still too much. You don't need it. If you can force yourself to eat tiny portions and the right kind, then do that now. You have to anyway to get approved for surgery. Her husband joined in so she ended up with the balloon, which is temporary but effects the same thing while it's in.
If I were to push something on someone who desperately needs to lose weight, it would be the balloon, but that was not a walk in the park. My friend didn't feel hungry at all. To the point she didn't want to eat at all. She'd drink a protein shake and her Dr. kept saying she has to force an egg or tuna or chicken. She had trouble drinking enough water. There is a big vitamin regime. This is a person with a lot of willpower and discipline. Who went to all the appointments and listened to what they said. The weight was NOT falling off as expected and she had to increase her workouts to often two hours per day.
The friend that did the sleeve also says it's not easy at all. Apparently some people have been saying she took the easy way out and she made a big thing on facebook on how that is most certainly NOT the case. It's a TOOL, and only good as long as you use it correctly.
I remember many times she had strange cravings like pickle juice, she couldn't get enough, and it would turn out to be an imbalance in her body due to these tiny portions, everything has to be just so. She went to the ER twice. I am not exaggerating about the lifelong battle to get enough nourishment.
You have to ask yourself, the way your sister eats now and why, is she going to restrict herself to 3 oz meals of exactly the right sort for the rest of her entire life? From what you wrote, the answer is no, she is not.
If she can temporarily to go from severely obese to merely obese, or maybe a little better than that, there is no gain to a permanent attempted restriction, only danger.
And I didn't even mention the complications. It's very risky, should not be used the way it is by so many who see it as a quick fix, even for diabetes type 2.