There will be many more skilled than me in this area, however, I can provide some guidance having gone through this a couple of times.
Is there any formula / rule of thumb to determine how reasonable the landlord's price is?
In theory yes, however it is often best to agree with the landlord a reasonable rate and move forward. See link: https://www.lease-advice.org/calculator/
You can of course try to negotiate the price, especially if doing two. However, you may find little flex as they will not want to lower prices for other future leaseholders.
Would my sister need to engage a solicitor, or could she just tell the landlord to go ahead if she's happy with the prices?
Definitely 100% engage a lawyer. There are intricacies to this process and getting the legals right and registered correctly on Land Registry is crucial. Not to scare you, but I had one where LR queried some wording in the extension, which meant we had to start again effectively, in total it took over two years...they are not all like that, but they are usually fiddly in some respect. By the way it is normal for leaseholder to pay freehodler legal costs.
Is there any possibility that the proposed changes to leasehold property laws I keep hearing about will affect my sister's position? In other words, is there a strong argument either for getting on with it ASAP or for waiting for the law to change?
Yes I believe so, however I would suggest this depends on how long your sister's lease currently is. If it is getting close to 80 years, then get it done or at least start to...as below that the price is usually much higher (marriage value).