Just take a little time and think it through before you report OP, I’m not trying to guilt you but people on here will always suggest reporting her because ‘It’s the law’ but I wonder how many people saying that would actually report their own sibling themselves based on just the limited information you have.
just think about the repercussions of it before you do anything, your mum’s not going to be around forever, and you’ll be left with the fallout and legacy of whatever decisions you make now, make sure it’s the right one.
If it was me I would print out the forms, go and see her and say, ‘look, mum has let slip that you’re taking money out of her account, I want to give you the benefit of the doubt that it’s all above board but you’re not giving me anything to put my mind at rest, I’ve got the forms here to report you to the OPG for mishandling the money, so unless you let me access EVERYTHING and see what’s going on, I’ll file the forms in and let them deal with it.
if she sticks to her guns, by all means report her, she’s had her chance. But if she comes clean, then look at the information she gives you and see if it’s within the scope of a POA or if you think she’s pushing it and sort out a plan going forward, if the money taken can be rectified you could even ask to be a POA too so you both have access to everything.
It could be that your mum is genuinely asking her to do these things, but there’s a difference between taking money out of the accounts as you’re allowed to and setting it to one side (in case you need it) and gifting it to people that are pissing it up the wall, either way I’d give my sister a chance to come clean.