Let me confirm the facts please.
There is a valuable book which belonged to your mother given to her some years ago.
The book would now belong to the beneficiaries of the estate, you and your sister?
Your sister has sold the book and pocketed the money?
There is a note in the book which belongs to you and she has also sold that and pocketed the money?
She doesn’t know that you know about the sale or money?
Is that the situation?
The first thing to do is obviously speak to your sister about it because this is theft and is a police matter. I appreciate that you might not want to involve the police and that is admirable but if you don’t, then your sister could well get away with this.
You would need to get a civil injunction to get the return of the items from the museum and the museum would net then need to bring a civil claim against your sister.
My first port of call would be my sister and the second port of call would be the police.
The police are unlikely to do anything about the book which belonged to the estate and they are likely to say that is a civil matter but the note was yours and that would appear to be beyond dispute
Unfortunately, legal proceedings are not cheap and are never without risk and although you can inquire of solicitors as to whether you could get any to do this no-win no fee, it seems unlikely and you have said that you’ve already spoken to several.
Can I clarify anything for you?