Hello and welcome to the forums.
I actually quite like Suze Orman's recommendations on this topic. On one of her podcasts earlier this year, she addressed it specifically for a child who was worried about her mother's irresponsible spending.
Basically, it means one uncomfortable conversation setting the expectation now. As this is your H's relative, it would be your H to have this conversation, in person.
"Bob, ther's an important topic we need to discuss today. In the past, when we have discussed personal finances, it has been a bit uncomfortable, however there are two things I would like to share with you.
First, you are my brother, I love you, and I am willing to help you out now with any planning or budgeting to help you get on stable financial ground. According to (pick your favorite credible source) men our age should have $xxx,xxx to fund our retirement until yyy. Not aking you to share what your situation is, however if you want help planning to get in a great place financially, I am here to help. If you are in a stronger place than I realize that is awesome. However, I worry sbout you, bud.
Second, Bob, I will not be in a situation to help you out in the future, by giving you money or paying for things. I completely realize you have not asked for help. However, it is important that you know this."
You can work on the scripting to make it your own, but this has to be addressed. Rinse and repeat as money topics come up.