83 year widower old with large IRA for benefit of kids and grandkids. Is the tax hit worth it to convert some to ROTH for benefit of longer tax-free growth?

Could use some advice to see if I'm thinking about this correctly.

My brother is an 83 year old widower with Alzheimer's. We both did well in our careers and have significant retirement savings so I'm not in his will and he's not in mine. But I'm his power of attorney and have been managing his accounts since his illness for the eventual benefit of his two adult children in their 50s.

He has a traditional IRA of about $5 MM. This results in a $300k RMD for 2026. Combined with his $53k in social security he's at the low end of the 35% marginal tax bracket. He has about $300k of space in that bracket before he hits the 37% bracket. He and kids are also in a high tax state (MD), about 8.5% marginal combined state and local.

He has more than enough non-IRA assets to cover his care for as long as he lives -- the IRA is purely for his kids for the benefit of themselves and the grandkids.

I'm considering converting some of that IRA to ROTH, perhaps as much as $1 MM this year and $1 MM next year. In doing so, I'm mindful of the fact that neither of his kids are in as high a tax bracket as he is. Kid 1 is in the 24% bracket, kid 2 is in the 10% bracket. Whatever the kids inherit in the traditional IRA will need to be emptied based on the 10-year rule, so if I leave it all traditional $2.5 MM each will mean minimum RMDs of $100k but more likely evenly spread about $250k/yr over the 10 years. For kid 1 that will be an effective tax rate on the distribution of 30% federal. For kid 2 it's an effective tax rate of about 21% federal.

Unless I expect income tax rates to rise, it's hard to justify paying a bigger tax hit to convert to ROTH (taxes would be paid entirely from non-IRA funds).

The factor that still inclines me toward converting is that kid 2 is an awesome person, devoted to her father, great mother to her kids, but has never been good with money. Has no savings and very little income. I'd like to position her well for her own retirement. Moving $2 MM to ROTH means her inherited traditional IRA is $1.5 MM and RMDs are $150k per year (significantly more money than she earned in her highest earning year). I know she'd still have access to the ROTH but she's self-aware about her financial difficulties and will follow a plan. She and her brother (kid 1) have a close and trusting relationship. My hope is that that she will reinvest some of those RMDs across the 10 years but that even in a worst case that she doesn't, the $1 MM in her inherited ROTH can sit untouched for 10 years. At 7% it should be $2 MM tax free 10 years later, which should be more than enough to provide her an income stream throughout her retirement.

What's the best way to think of the tradeoffs between the opportunity cost of the roughly $800k in federal state and local taxes he'd pay to convert $2 MM now versus the value of uninterrupted tax-free ROTH growth for the full 10 years. Assume for the sake of argument that both kids will leave any ROTH funds untouched for the full 10 years because I believe they will and otherwise this becomes even more complicated.

Thanks Reddit. I'm all ears.