I have been appointed POA. How do I inform siblings of this?

My Mom has given me authority as her agent on her Durable POA. She suffers from dementia. I have 3 siblings. 2 of which are in denial and are also engaging in a war of sorts over our mother. To date, those 2 siblings have done little FOR her but are quick to take FROM her (my 50+ year old brother has borrowed $12,000 that I know of from my mother over the last 3 years and has not paid it back). My other sister and I are working TOGETHER on behalf of my mother. But here is the kicker - my oldest sister and brother think that by force they can control the situation. Neither know that I have POA. So my question is - do I need to inform them that I have this "power" and if so, what is the best way to inform them (also from a legal standpoint)? They are semi-bullying my mother. My mom is afraid of them and historically neither have done much for her and my brother has a history of making poor decision regarding the finances of other and over his finances. He's responsible for one grandparent dying nearly penniless, a great aunt was the victim of elder abuse and my brother's response to that was that he didn't have the finances available to hire a lawyer to address this...

Just need to know what I need to send to these yahoos, what I should say to them, and how I need to do it. Obviously, I need to find legal representation in my Mom's state of residence.

Thanks all!