I have written in several times for advice. Up until 7/9/18 My Sis was POA and Trustee of mom and dad. Well a couple months ago Mom had been calling, crying begging for help. Sis had been stealing Dad's IRA, Social Security, Rental property checks, Not allowing mom and dad to BUY groceries, getting them food pantry food which is mostly sugary foods when dad is a diabetic. My brother wants mom and dad to be peaceful and not have a stroke. My brother and I took Mom and Dad to lawyer to change trustee and POA. Brother made jail threats and we were thinking that Sis and Bil were scared enough to comply. Last week I took mom and dad to bank A to close accounts and open new. I told bank mgr what was going on with Sis as POA. We left bank A and went to bank B. Bank B was way to busy and mom and dad were getting stressed so we left. I went back to bank B today and found out Sis wiped out all dad's money but $5. Then I proceeded to open new accounts. I drop dad off at home and I am in a store and Bank A calls me to say Bil was just there with my dad (confused) trying to get dad to withdraw all his money. Luckily the bank mgr saw dad was confused and kinda squirted around irate Bil. I raced to bank the mgr stayed after hours to have me put account money into my name only with mom and dad as beneficiaries. Is what Bil did legal? I think Bil and Sis will now try to do this with Dad's rental property. I think you will say get guardianship, right? How long will this take? How do I stop the stealing in meantime? I am so frantic I can't sleep.
So, calmly call the police!
If it helps, write down the important bits first. You want a timeline of [date] [what happened] without judgement or emotion.
But for your parents to be kept away from the stress of this situation, there needs to be a very clear line drawn; and the lines drawn within the family evidently haven't been clear enough.
We're not talking about your parents never seeing your sister again, or anything distressing for them. We're talking about getting BIL out of everyone's hair. Got to be a good thing, no? - and it sounds as if the bank manager will provide a statement so any questioning of your father can be kept to a minimum and very gentle.
But what BIL is doing is elder abuse, and that is criminal. He is trying to steal their money! If he were caught trying to blow open a safe in their basement, nobody would think that was borderline, would they? So I think I'd also call the police for advice.
Is the lawyer you have been using a certified elder attorney?