The contract will not be void
There is a disagreement between you and your sister as to the price to be paid for the house. However, the vendor is an innocent party in that disagreement and a contract will not be made void if it injures innocent parties.
Under the contract, you and your sister are obliged to pay $600,000 jointly and severally. Neither the vendor nor the court cares in what proportions you pay so long as you do - it could be $0 from you and $600,000 from her all the way to $600,000 from you and $0 from her.
If you break the contract you will lose your deposit and will also be liable for the vendor's losses to the extent that they are greater than the deposit. For example, if the deposit is $50,000 but the next closest bid that can be converted to a sale is $520,000 then the vendor has lost $80,000 which will come first from the deposit with the vendor being free to chase either or both of you for the $30,000 balance.
If you have a contract with your sister about what your respective shares are and what you are each obliged to pay then the court will hold you to this. However, if you just have an 'agreement', which is what is presumed between family members, then you have to sort it out on your own.