If you all share equally in the ownership then yes, you are all equally responsible for the maintenance and upkeep.  If they will not pay, you and your sister need to keep very good records on all the funds that you are expending with estimates and receipts for repair.  When it comes time to sell you need to ask for reimbursement for what you put out.  You should consider having the proeprty appraised as is and then after the repairs so that the increase in value can be calculated as well.  Good luck.