"So, what should I do?"
All we can do is acquaint you with the tax rules regarding the mortgage interest deduction. Since your father is a co-owner, and since per 26 US Code 280A "personal use of a residence" includes use by a family member, it appears that you and your father each have a lawful claim to the deduction. Technically in this situation you should each claim the portion of the mortgage interest that you actually pay. This is an issue that you and your father have to work out between yourselves.
26 U.S. Code § 280A - Disallowance of certain expenses in connection with business use of home, rent...
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
