How do I take charge when one sibling manipulates our Dad with his children involved as well?

as far as I know, POAs an be written differently. Some are in effect the moment the person signs it.

I have a slightly different take.

While I agree that simply making "bad" decisions (or ones that are different than those we might make in the same circumstance) does not mean that someone no longer has capacity, this MIGHT be a sign that your father is suffering from memory impairment or other cognitive decline. The POA document itself will state how "capacity" is to be determined -- sometimes, it states that two physicians must put their finding in writing. Sometimes, there are other triggers. However, as POA, if you feel that your father might have become incapacitated (or be on the way), you could consider taking him for a neuropsychological evaluation by a properly credentialed professional. This is an in-depth look at memory, decision making, and other cognitive measures and is an accepted way of determining capacity. If the evaluation comes back that your father still has capacity, then you will need to find a way to work within the reality of the situation, for now. Alternatively, you might learn that you ARE able to invoke the POA and take charge. As I caution all of my clients, be sure that you take charge without taking over. There is a fine line...

Sheri Samotin, CPC, PDMM
Author of Facing the FInish: A Road Map for Aging Parents and Adult Children

You say you want to "take charge." But, right now, if your dad is still of sound mind and able to handle his own affairs, then your POA does not become effective, and you (unfortunately) are not authorized to take control over anything. You may disagree with what your dad is doing, but the POA does NOT become effective, until the parent is incapacitated (like in a coma or seriously suffering dementia---not simple bad decision here & there).
IF you believe your dad qualifies as "incapacitated" and want the POA to kick in, you better work it from that angle, contact the family physician and document what you feel is proof that your dad is incapacitated.
If you don't want the responsibility of being POA then tell your dad to name someone else. You can still be Executor. The POA ends when your dad dies.

i also have a different spin on this and you may not like what i'm saying but i have to say it. you may be the responsible money child who always handles the business so your father may be trying to make you happy by keeping you in that role. he may be trying to keep the other sibling happy in a different role. he may even enjoy the other sibling better. many men, around midlife, really get sick of being the money making machine and they change their priorities. they hang around people who know how to enjoy life in a similar manner that they do. unless you're willing to ease up and drop the role of financial protector you will probably have to live with the situation as it is.

Your father's relationship with his son has developed over a lifetime and is unlikely to change. Keep careful records, bank online if you can, keep images of every check your brother got. Warn him that if dad goes into a nursing home, Medicaid will count every penny as a dollar-for-dollar penalty, and your brother will have to pay the nursing home bill, because Medicaid won't. Medicaid looks back five years on the financials. You can remind Dad about this too, but it may not change what he is doing. It might help to have Dad sit down with an estate lawyer and have a long PRIVATE conversation about how to protect himself and his assets. You go with him, but expect to spend time in the waiting room while they sort things out. We did this with mom, met with the lawyer, then he asked us to step out. When we came back in, they had made the right decisions and came up with a PLAN in writing.