How do 18-25 yr olds on ACA plans affect subsidies? Should they have their own plans?

Hi all — I had a talk with my stepsister yesterday about her health insurance situation and came away with the impression that she is being let down by various financial advisors and I am trying to figure out how to help her. She is self-employed as a therapist; her husband was a school teacher for many years and they always had insurance through him but he retired last year (though is too young for Medicare) so this year they bought a plan on the individual market through a broker. They have three kids — one in college, two others who have graduated but are under 25 and are still on their health insurance.

The amount she is paying is fairly eye-popping and I do not believe it reflects the tax credits they should be getting, especially what with the COVID-era expanded ACA subsidies being extended through 2025. Because she went through a broker, she’s not actually sure if her plan was purchased via the exchange or not (I feel like it would be fairly serious malpractice if the broker didn’t do so). I am trying to figure out what the subsidies should be but I am confused as to how you calculate your family AGI if your plan includes adult children who are not part of your household for tax purposes. Beyond that, since those kids are both working fairly low wage jobs right now, would it make more sense for each of them to buy their own insurance through the exchanges? She was operating under the logic that often holds with group plans, where “family” plans are the same price no matter how many kids you have on the policy, but later she realized that may not be the case for individual market plans.

As another side note, she was under the impression from her accountant that her premiums weren’t tax deductible, which makes zero sense to me. She is full-time self-employed and the family has no access to a group plan through an employer or anything else, so even if her broker screwed her and she can’t get ACA subsidies this year, she should still be able to get the self-employed tax deduction, I think.

Thanks in advance for any advice or information.

ETA: Stepsis is in her late 50s, husband in early 60s. Living in Western New York.