You too will go broke helping him. "getting him back to healthy" will not happen. He is the perfect victim actor. He moved from Mom to you/brother. You too are an enabler. He is not your responsibility. You have a choice. Do it or don't do it. You don't sound like you're rolling in dough. YOUR retirement comes first and it sounds like you are far from prepared for it.
Tough love. Your DH is right. I would not go behind his back for your BIL.
I can only go on the bits I have picked up from previous posts about your family situation. Could it be that you gain a sense of purpose for being the provider in the household? Or could it be your Catholic background fosters a lot of guilt over such decisions? I would say do what you need to do to help him find another "provider" and move on.
Many people that retire do not do it by choice but are forced to by health problems. Any day something could happen making it impossible for you to work and you would have to survive on your assets and SS. Can you do that? When you lend $ to people they grow to resent you in the long run. If you force someone into MH treatment it does not work. Do not be a burden to your kids in your old age by giving your $ to him. No, no and no!!
Also, I'm not sure if anyone said "eep" about the idea of lending BIL money without DH's knowledge. I think that is a truly dangerous idea. From what you've mentioned about BIL asking DH to drive BIL to the hospital on a day when he truly should not have been doing so, it sounds like your DH's health is not BIL's priority. Pretty fair bet that he won't put your relationship with your partner all that high on the scale of things he's worried about either. "Well, she lent me money before..." i hear him say.
I sound a bit adamant. I've had a leaning nephew, a leaning brother and a kind of leaning BIL, and in my experience giving or loaning them money accomplished nothing good.
OK, so you guys think I should definitely give him the money, right?
I'm getting clear signals. No money. Look after myself. Got it.
"Do any human beings ever realize life while they live it--every, every minute?" Emily Webb, Our Town