How do I take over legal guardianship and power of attorney from my younger brother over my mother?

If he in fact has guardianship he is responsible for her care totally. I don't know which state you are in but your state laws will lay out exactly what he is responsible for doing.
With the guardianship I had for my wife I had it all. I was guardian for her person and her estate. She lived at home with me. I saved her SS expecting to need it in the future for her institutional care. I paid for everything else. That included shelter, food, most clothing, transportation, meals out, her protective underwear, bed protectors, etc. The only money I took out of her account was for her intertainment ( think gambling) or special clothing like her LaMaze bras. I paid for meals out.
I had to turn in to the court an annual balance sheet of her money, expenses, and inventory of her possessions, like jewelry automobile.
If he is doing nothing I think you have good grounds to file for guardianship. But that should take a certified elder care attorney.
In this state if he only had guardianship for her person then another person could have been appointed as her POA for her estate or vise versa.
If he has guardianship over her person and estate then there is NO POA.
Whatever your wishes are, you need to retain a certified eldercare attorney and file the necessary paper work with the court. This is no easy matter but it may be necessary in your state.
Additionally , here when the guarded person passes away, the guardian needs to file a petition to have the guardianship terminated. At that time an executor needs to be appointed for the estate.

Ihope this helps you and I wish you the best of luck.

There is no more POA, guardianship took its place. You would need a lawyer and money to override his guardianship.

You can’t just take over POA and guardianship. And POA is never “over” someone, it is for someone. Your mother cannot assign a POA anymore since she now has a court appointment guardian. If you want to become her guardian, you have to go to court for that and convince them that it should be you and not your brother.

Your mother makes the decision as to who she wants as power of attorney and the court decides legal guardianship. Power of attorney is not over someone. POA gives one the ability to act on behalf of a person.