How do I transfer POA from one of my sisters?

Your sister cannot transfer her POA to you. It can only be granted by the individual himself and must be transferred by your father since it is granted by him legally (if he has dementia he probably cannot change it) and the only other choice would be a guardianship of your father which would give you the power of attorney. You might consult with an elder law attorney through legal aid clinic about the costs involved, which can be thousands. There are several threads concerning this - you can try searching in the box at top of screen too.

Only your dad could have changed his POA--but if he is too far gone with dementia he can no longer make that change (legally). You or someone must pursue Guardianship at this point. Be aware, the judge may choose a different Guardian than what the various siblings might have wanted. Also it does cost a good amt of money to file for a Guardianship court case. Might be easier just to keep the old POA in place and work around the issues.

Have you discussed this with your sister to see if the two of you can work out an agreement?

Carer's Allowance? Are you in Ireland? I'm a little out of my element here, but have reviewed the forms. The person being cared for is supposed to sign the form. But as your dad has dementia, it would be your sister, with POA signing it for him. It doesn't appear that you need POA to be the 'Carer'... but your sister would have to approve you to be.