Theft and family - part two.

2 days ago I posted about a lot of money being stolen from my nanas account and I'd like to update and thank the lovely ladies that commented and ask advice again.

I called the cops and he came out yesterday and came back today, after talking to my sister the cop thinks it's her or she has known about it and has benefitted from the money. he has asked us to make a decision before tomorrow on Wether we want her arrested and questioned along with her ex, or if we only want her ex arrested and questioned, he personally wants to go tomorrow and arrest both of them and is willing to do everything he can to get them sentenced for this. After looking at the bank statements of her 3 accounts, there has been $44,598,87 taken over a period of 3.5 years and she has been left with $8 to her name! The advice I'm after is do we allow him to arrest both of them or only him? And how do I convince my nana to allow that (she doesn't want to get her into trouble, but wants to know the truth). I'm heart broken and I've found out today there is only 2 out of 11 bills being paid, so if anything were to happen to her house, her life, or her boiler she would have to find the money from somewhere to pay because my sister hasn't being paying them! I don't know what to do for the best.

**UPDATE** the cop has been back out and the 3 of us decided to do all we can to find out who has taken it. While he was there my sister turned up in a mood, she had a go at my nana, infront of the cop, she doesn't know that we suspect her but knows something is going on. He took me outside and told me if she kicks anything off to call him immediately and he will be straight back to take her away (she stormed out without saying goodbye), if she doesn't to ring him Sunday morning at 7 am and he'll come back out when she's not there and get a statement from my nana and us, in the mean time we need to get all her bank cards, statements, receipts, bill letters, and any cash my sister has belonging to my nana back to her house so he can veiw them all, he also wants copies of all statements with all the money that has been withdrawn without her consent and knowledge highlighted. Is there anything else you ladies can think of I need to do/get to help with the investigation?