as a co-owner of the property, you can use it and build your home. the title though will remain to be in the name of the current registered owners but you can have the improvement tax declared only in your own name.
you can leave the title as is for now. or you can purchase your father's share now, but it is correct that your sisters have to act through a guardian of legal age. do you believe there is a real risk if you have co-owners of the property? (the answer depends on your relationships with your father and siblings). once your sisters reach 18, you can then transfer the property to just you.
one issue you may face though is the application of building permit because it requires consent of the owners, but I believe a little convincing can get you through this possible hurdle.
if you believe there is risk in keeping the property co-owned, then I recommend that you go by the book and have your father file a bond to be the guardian so it can be properly sold to you. this trust agreement, if there is risk, is not the path I would take.