How broad is my authority on initiating actions I'm confident my brother would support?

Your profile indicates you're Trustee as well as proxies under springing and medical POA. These are the sources for what you can do.

Some Trusts provide for periodic and/or annual disbursements to family members, especially from asset portfolios. You need to review the Trust thoroughly, and even better, ask for interpretation from the attorney who prepared the Trust if and under what circumstances disbursements can be made to the children.

Given that he's in need of a high level of care, and if there are any provisions for disbursements to heirs while he's still living, I'm thinking that a properly drafted Trust would first provide that funds go to his maintenance and needs. That's an aspect you need to verify.

No sorry, that is not within your authority as a POA. Its not his responsibility to care for nieces and nephews.

NO, If you’re financially secure, give your children some of your money.

No you cannot.

Absolutely not.

No it isn’t within your authority to give johns money to your children and your sister-in-laws children. If he’s no longer competent and of sound mind, you cannot take it upon yourself to give his money away.