if this was inappropriate surely the solicitor wouldn’t have complied.
Sadly, I'm afraid this is a mistaken view. Solicitors get paid for executing their clients instructions.
To be fair, most solicitors will accept instructions in the first instance. How could they possibly know in advance whether there has been shady behaviour?
The solicitors would be looking for a case to be made against your sister. Sounds as if they are now looking for collaboration of her position.
Unfortunately, you can't be sure whether they are the kind who would drop the case if your don't find anything suspicious. They do that by suggesting to the client that there is nothing more to be done. Sadly, there are always other solicitors happy to take money for sending letters.
I would encourage you to get your own legal advice, in how you can best close down his enquiries. Not necessarily even writing any letters, just to advice how best to respond.
Wanting the last seventeen years of bank statements certainly sounds vexatious. There is in fact no way to compel you to answer any letters from his solicitors, without a court order.
All the best.