It seems like you could get a POA and then just transfer her money out of her account as soon as her SSI is deposited. Do you think she could let that happen, or is so so irresponsible that she'd try to sneak in and grab the money first?
Id just be very careful to make sure it doesn't look like you are actually taking her money. Send it to a separate account and pay all her bills from that account, and perhaps also have something written up explaining the situation (signed by both of you), just in case.
You could also ask her to hand over her ATM card and checks. (Or perhaps get her a new card for a new account into which you transfer a monthly fun-money allowance, so she isn't without cash.)
None of this is fool proof, is she really wants to get her hands on everything, but it would make it more difficult for her to do something impulsively, and easier for you to find out quickly if she does.