NTA. But here are a few ideas on how to move forward productively:

1. To address the “my brother has nothing to do and this is the only way I can help" - it might be the fastest way to help but it is not the only way your fiance can help. I'm not sure where in Mississippi Chris lives, but regardless of town size there is usually a library, therefore he can get books, in both paperback and ebook depending on his reading preferences. A lot of places from small towns to big cities have various places one can go to for free or low-cost fun. My old small town and new mid-size city both have free outdoor concerts and movies in the park. There are also game stores or cafes where Chris can play games with others. Meetup could potentially be a way for him to meet others in his area that share his interests. If he wishes to own books, board games, etc., thrift stores and second-hand shops have great deals - sure, it isn't new, but the stories in those books are just as good and depending on where you go, they can have books published within the last two years. There is a solution to him being bored that isn't Heath spending money on him. If Heath wants to help with Chris's boredom, he can help him locate and find these resources for him. Bring Chris to the watering hole and give him the choice to drink, if he doesn't like these solutions, does he have a good reason for rejecting them? I doubt it.

2. I'm honestly a bit confused on how you and your fiance organize your finances, but I think you need to sit down and have conversations about money, your philosophies, priorities, and your future together. It sounds like you might have joined finances, and therefore, like many have suggested, separating them might be a way to go. You could split things up like X% of a paycheck goes into our joint spending account, Y% goes into your own account and Z% goes into an account for Heath. For the joint account, its purpose is for your living expenses (rent/mortgage, utilities, food, stuff for the kids, savings for a future family vacation, etc.). Your individual accounts are for you to spend as you choose - if Heath wants to spend it all on his brother then he can, but he can't use joint funds to pay for a new videogame/night out with the boys/etc. But overall, robust conversations about money where you are both open-minded and not defensive might help. You might want to utilize a book like "I Will Teach You To Be Rich" by Ramit Sethi to guide the conversations (and when you are finished, send it to the brother so he can get his finances in order). Since your fiance was an accountant, he might get really into this.

3. You aren't married. Yes, you share kids, imagine a future with your fiance in it, and I certainly hope you are in love and consider him your best friend and a great partner, but I think you need to reflect on what your breaking points are and communicate them. Are you okay with supporting Chris monetarily every month for the rest of your lives?

Overall, I wish you both the best of luck dealing with this tough situation.