My mom and stepdad have been married for 30 years and together for 40. Second marriage for both. I am an only child and he has four boys that took my mom and dad to court and got guardianship as soon as they found out he has money in stocks that are substantial. The boys are all wealthy in their own right but decided to take over the parents’ lives. It has caused horrible consequences to my mother and father. The stress has been horrible and to this day my mom takes care of her husband twenty four seven with my help. The guardians want to control every move and are building a narrative against my mother.
My heart goes out to you! Believe in Karma - what goes around comes around. There must be a special place in Hell for people like we are dealing with! Stay strong and get a Good Family Law Attorney.
As for your thoughts about them taking over, guardianship does take over their lives to keep them safe from predators and their own personal safety. They might even remove them to skilled facilities which means selling the home and placing their assets in an account set up for them. You see it as "horrible". Guardianship IS a lot of work.
Many people write about taking care of a parent only to have someone take over their lives. Make nice to the guardians. Ask if they can pay you for caring for them, however not as a slave 24/7. You need a personal life as well, away from your parents so that you can thrive.
Are you sure it is guardianship and not something else, like PoA? To get guardianship for their father they'd have to present their case in court. Is this what happened? If your Mom was her husband's PoA then none of this would have happened -- if she was a competent PoA.
Whether they (or one of them) are his PoA or guardian, there is legally accountable for those roles. If you think they are abusing him, then you can report it to APS or hire an elder law attorney. From my own personal experience it was not easy to convince my Mom's annuity managers to allow me to take control of it -- I had to sign an afidavit and provide doctor's letter of impairment, among other things.
I think in the absence of information and understanding you are making assumptions about what is actually happening out of fear. This kind of division isn't uncommon in blended families where there are adult children... they tend to "reclaim" their parent.
You can take your Mom to an elder law attorney, or to her bank and start thinking about whether you need to help her protect herself financially. When her husband passes, all his assets will flow down to her, even if there is no Will.
If your Mom doesn't have a PoA (or guardian) then you should try to get this put into place so that you can help manage her medical and financial affairs.