Surely Shannons would do a valuation service.

I’d agree with that 25-40k as stated earlier,depending on the quality of the restoration. It sure looks the business.
I guess for you it all comes down to whether the sister is happy or not. Family dynamics pretty much outstrip anything else. It’ll be hard for you to enjoy it if she feels like she got the short end of the stick. Looks like you’re on the right track though with an independent valuation.