She's talked to each teen privately and no one "knows" anything about it. The kids' friends are not suspect at all (they've been ruled out completely due to the proximity of the money and opportunity). It's most likely an "inside job" .
She knows that putting all three on "house arrest" will probably not solve anything. Should she set a trap? She'd rather not have to lock up her purse in a bank vault each night!
These kids are sending the message that they want more money than they have. I would suggest that house arrest will do no good. They appear to be standing firm on not coming forth with the truth.
Your sister could confront them together and say the following: "We all know money has repeatedly been stolen from my wallet. It hurts me and disappoints me a great deal that no one will tell me why this money was taken. I'd like to think I would be understanding enough so that we could calmly talk about why anyone in this family needs more money. The three of you need to discuss how you want to reclaim your position in the family as people who can be trusted. You have three options: You can tell me as a group what went on and why, and I can try to understand and then forgive you. You can return the money stolen, installment by installment, by placing it in an envelope on my bed when I'm not home and we'll consider the matter closed. Or you can type me a note explaining what happened and why and if you don't want to talk further, that note will end this situation ".
Your sister can conclude with: "I am not going to behave like I'm in a prison where I can't leave my possessions/money around for fear they might be stolen. If money keeps being stolen, I will know that something radical must be done because you cannot and will not act honorably. Thank you for listening. I hope we can put this behind us and trust and respect each other again".
Let me know if this helps.