How much should I help an adult sibling who is struggling financially in retirement as a result of making poor decisions?

Surely, we can only speculate from out here in the peanut gallery, every case is different.
I would suggest you go with what your heart tells you, but not at the expense of your better judgment. You have to live your life, and deserve to reap the fruits of your labors and peace of mind; that they may have squandered theirs, as you imply, isn’t your fault or responsibility. Take on as much or as little as a clear conscience allows, uless you sense the effort to be too great a drag or simply futile. You didn’t put them in that hole. 

Help might be as much about giving guidance and making connections to other sources as it is about handing out money. I’ve always been a believer of the old maxim;
Give a man a fish, he’ll eat for a day; teach a man to fish and he’ll eat for a lifetime.

Not much financially, in my opinion. With out going into detail, I understand the guilt involved with being financially secure and seeing a family  member struggle financially due to poor decisions on their part.  I don’t think you should have to give up what you’ve worked for because of someone else’s poor planning and decision making.  I would offer support in the form of helping them find resources to supplement their retirement income and find affordable housing, maybe invite them around for meals more often.... Unless you’re a millionaire or something...in that case I’d like a pony, STAT. 

Many years ago, I had a "ne'er-do-well" sister-in-law whose income never seemed to match her outgo.

It was suggested to me at the time that my wife and I sit down and decide what our maximum assistance level would be and then stick to it.

That turned out to be both reasonable and practical and allowed for a clear understanding of important issues that should govern such a relationship without the burden of the emotions which an emergency might cause.

This is really tough because my husband is going through the same thing with my father-in-law.  You can do what you think is right at the time and still end up feeling guilty and second-guessing yourself if it turns out badly.  In the case of my father-in-law, none of the help my husband offered was good enough.  My father-in-law tells people exactly how he expects them to help him and nothing else is even considered with him.

I have another time bomb waiting to explode with my own mom.  She's 58, has only $5057 in her 401(k), and has only been working six years so she probably won't get much for social security benefits.   I don't know why her 401(k) balance is so low unless she's only been doing something like adding 1% of her income per year.  Ugh, why do people do this?

Anyway, the best advice I have is if you don't have the money to help her out you just don't.  You can't go broke or bankrupt yourself from trying to help someone else.  Your brother or sister's just going to have to find affordable housing.  My father-in-law makes $850/month with social security and has no other retirement income so he had to move into state housing which he wasn't very happy about.  He wanted to move in with us and not pay rent so he could keep buying new cars and toys but my husband just said no to that.  It didn't turn out well.

You're just going to have to be really clear with your sibling about how you can and can't help and not let them talk you into something you can't do.

That sounds like my sister. She made many bad decisions and still does. She now lives in a crappy house in the 'hood, and continues to spend what little money she earns on drugs and extravagant non-essentials. She's not getting a dime from me and she knows not to ask.

How much heart do you have?