I am making two assumptions here.
1) Your name is listed on the deed as an owner.
2) Your name is on the mortgage (if any) making you legally liable for the loan.
will assume the cost for insurance, HOA, taxes, and repairs.
None of that is tax deductible. Not a penny.
For your taxes only, your cost-basis in the house is what *YOU* paid towards it's purchase. That's 25% according to you. But this *does* *not* *matter* at this point in time for your 2019 tax return. It will not matter until the tax year one of three things happens in your life.
1) You convert the property or any part of it to business use (such as a rental).
2) You sell the property.
3) You die.
The only things you can claim/deduct on your 2019 tax return for this property is the property taxes and the mortgage interest that *you* actually paid. For taxes, it does not matter if you paid 100% of those deductible expenses even though you only have 25% ownership in the property. If *you* pay it and can prove it if audited, you can deduct it. Period.
Now for tax purposes based on your scenario with "ONLY" the information you've provided, any transfer of ownership percentage from your sister to you is considered a gift from your sister to you. If the value of that gift exceeds $15,000 in any one tax year, then your sister *NOT YOU* is required to report it to the IRS on an IRS Form 709 - Gift Tax Return.
Now the name of that form is a misnomer. There is no actual tax assessed on anyone on the value of the gift given, provided the gift given by any one individual to another individual in a tax year does not exceed a total of $11.4 million dollars over the giver/recipient's lifetime. But if given more than $15,000 in a tax year, the giver (not the recipient) is required to report it to the IRS.
