Be careful, as the house is being sold and will be done by solicitors conveying this it is easy enough to check the price by ringing up and asking the solicitors of this.
I know it is difficult but the fact that she has got a buyer and has sold and you have a percentage of the house rather than a fixed price means she will be "loosing" out just as much as you.
I think actually that it was sensible to take a real offer rather than holding out for a potential offer and waiting indefinitely for a buyer to come along that was willing or even able to offer the valuation price.
It is no secret that the housing market is in termoil and crisis. One of my neighbours has had his house up for sale for 3 years and has had offers for only 70% of the "market value" and friends have told him to lower the marketing price - his resonse how will that help me as the only few offers I have had have been way below the existing price and in any case I haven''t even had any views now for the past 4 months - he and his wife are happily married and want to move by choice.