Can I buy health insurance for my mom and younger brother?

Hi everyone. I'm hoping someone can help me as I don't really understand how health insurances work. I live in California and have employer-provided health insurance. I've asked my employer if I can add my mom and brother to my plan, and they said no.

My mom is 52 and needs a handful of daily medications and frequent doctor visits. My brother is underage and is healthy. They used to have health insurance through my dad's employer, but he decided to retire early and now they don't have coverage. I urge him to look into the health insurance for themselves, and he dismisses my pleading. I should add that my parents file joint; he was the breadwinner while my mom was a stay-at-home wife. With what my dad worked this year, they do not qualify for free healthcare.

I'm willing to pay out of pocket for my mom's and brother's health insurance, I just don't know where to start or what to do. I fear that it may be more difficult since my dad wants to continue filing joint and declaring my mom and brother as his dependents. Any and all advice is welcome, please.