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I do not understand what you are concerned that your sister will do because I do not know what she will do in the US.
Are the POAs for when your mother is no longer competent?
Is she competent now?
I am so sorry to hear of this horrible situation.
If you are appointed under the POA then you will be in charge when if your mother becomes incompetent as you know. But you are correct in that she can be unduly pressured by your sister to give you sister money, for example. And that is a concern because it is elder abuse and hard for you to address if your mother knows what she is doing in the sense of understanding what your sister wants but is willing to give it to her when asked or pressured.
All you can do at this point is be very vigilant in terms of your mother's finances and perhaps try and not let her leave cheque books or cash around. That way you sister cannot steal from her or commit fraud or forgery. But also ask your mother to let you review all cheques that she writes and explain why it is best for her that you check what she is doing.
I don't know how the Trust works but make sure the Trustee understands your concerns.