Questions about guardianship and insurance

Hi everyone! My husbands HR department never answers the phone or returns our calls, so I’m going to ask here and hope someone can provide some insight.

I am sister to 3 teenagers who are 18, 16, and 15. I have legal court ordered guardianship of them besides the 18 year old. I am also on Medicaid until I turn 26 next year since I was in foster care until 21. I was able to get my siblings and son on my Medicaid since I have it.

However since I turn 26 next year I am going to lose my Medicaid coverage and will just join my husbands insurance since it’s much cheaper than what my current employer offers. Am I able to add my siblings to his health insurance as well? Including the 18 year who I no longer have guardianship of since he is now an adult?

Also to clarify, they have been in and out of foster care multiple times. My dad got custody of them back in 2017, but then lost them again in 2018. When he lost custody again, the state worker simply asked instead of putting them back into foster care, if I would take temporary guardianship and avoid the system altogether again. So i got temporary guardianship and then permanent guardianship a few months later. So I was never their foster parent. My husband and I were also unmarried at the time and didn’t get married until last year.

TIA for any help!