I'm Lucy, and I'd be happy to answer your questions today. I'm sorry to hear about your situation.
You're in a difficult position here, because when family members who own property can't agree on what to do with the property, it's always going to hurt the family relationship. The 70% majority can go to court and get a court order that the property be sold and the proceeds divided among everyone, based on percentage of ownership. That's usually what happens when joint property owners can't decide what to do with the house. The judge could also order a buyout, but that's more common if only one person has the means and the desire to buy everyone else out - if multiple people want to decide, then the property gets sold. With undeveloped land, the judge can literally split it into pieces and give everyone a share, but they usually won't happen if there is a house or other buildings on the land.
If you're not willing to go to court, then the 70% majority could proceed with whatever it is they want to do - but that's also going to hurt the family relationship if the 30% interferes. Joint property owners all have a 100% undivided interest in the whole. Unless the property is held by a trust or LLC that talks about majority, any one of you can make decisions that affect the property. The problem is, if I want to build a fence, and I put it up because I have a right to do so, but them my sister rips it down because she also has a right to do so, then I'm out the money I spent on the fence. So it's often better to try to work out a resolution that everyone can live with in advance.
The 70% majority could do what the 30% minority wants, which can also harm family relationships because of the resultant resentment.
You could also look into hiring a mediator to help resolve the dispute. Mediation tends to be less adversarial than litigation. If everyone will agree to go to mediation, that could be worth a shot. It wouldn't be effective with just a percentage of the owners.
If you have any questions or concerns about what I've written, please reply so that I may address them. If I did not address the specific thing that you wanted to know, it may not have come across clearly to me, so please restate that question. It's important to me that you are 100% satisfied with the service I provide. Otherwise, please rate my service positively so that I get credit for answering your question. Thank you.