So you would each be entitled to 27k if you sold. Less fees of maybe 1k for estate agent, legal fees. You have to assess whether or not taking legal action to force the sale is worth this to each sibling. Is this a life changing amount for any of you, assuming that you will only recover a portion of it after any legal fees to enforce, including the value of your time and energy in the process?
While it is against your parents wishes, would it also be their wish that you all fell out over it and brought stress into your lives? It is a horrible situation to be in but we can probably all quote cases like this in our families or communities and no-one is the winner except those collecting fees. It is so hard to walk away from what was probably hard earned and cared for by your parents but in some cases that is the sanest thing to do. It may be difficult with so many parties involved (then add in partners who might be too). And if you do, try to not revisit. Make the decision final.