Can a POA of one have POA over another?

???!

Yes it's perfectly legal!

Your brother has appointed you to act for him if he is unable. That's fine.
You have appointed your daughter to act for you if you are unable. That's also fine.

If you are asking whether this will enable your daughter to act for your brother...? No. The POA you have given her does not give her authority to act for your brother. It isn't like a chain of command.

If you have reason to think that you won't be able to take care of your brother, then your brother needs to appoint a new POA or a deputy.

I would also note that it might help if you understand the role of POA. The "power" is not something you have "over" somebody else. The somebody is *giving* you the power, or authority, to act for him. You hold power of attorney FOR someone, not OVER them.

Everybody selects the person they trust most to be their representative when they can no longer speak for themselves, your brother picked you and you were free to choose your daughter - I'm not sure why you think this might be a problem?