You never "declare" ownership of anything. You may have income to report, if something you own gives you income (like a bank account or rental property) or creates income when you sell it.
If you are on the deed to your parents' house and they have a life estate, then as long as they are alive, you can deduct any property taxes and mortgage insurance that you personally pay. If you don't pay the expenses, you have nothing to deduct. After they die and you have inherited the house, you may have a taxable event if you sell it, and you may have deductible expenses if you pay mortgage interest or property taxes at that time.
