1. If he says he is accepting an offer which is less than yours, then you may be able to do something through the Courts but would have to spend money on solicitors fees etc. If you won, these costs may or may not have to come from him personally (rather than from the estate) - depending probably on the judge's view of the case. Your ground for going to Court would be to prevent a transaction which would unjustifiably reduce your (& other siblings - who could be joined in the action) inheritance. You would have to do something pretty rapidly to get an order to prevent the exchange of contracts.
2. If he deliberately does something which reduces your inheritance then yes - he can be personally liable. Again, you would (if he failed to co-operate) have to go to Court & your costs should certainly become his personal liability. Make absolutely sure that your offer is confirmed in writing to him & that he cannot deny having received it. I assume he is selling through an estate agent & you will have to deal through them so it would be a good idea to make them aware of what is going on. They would have to pass your offer (& any others) on to him so make sure they have records to confirm he has had your offer. If he then tells the agent to accept a lower offer they won't be happy as their fee will be less; they should ask him to explain why he is not accepting the highest offer & make records of this.
3. Make sure your offer is fully backed by mortgage etc. & that the agent has details of this. Also, if you would have to wait to sell another property before going ahead he might be able to argue that accepting a lower offer was the right thing to do if the buyer could proceed immediately (i.e. had nothing they had to sell).
4. Of course, if he used agents valuation as a basis for agreeing a price with you it could be sold to you without incurring agents fees - thus increasing everyone's inherita