Treas. Reg. §1.163-1(b) provides that even if you’re not directly liable on the mortgage, you can deduct any interest you pay on the debt as long as you are the legal owner of the house (e.g., a deed holder).
https://www.govinfo.gov/content/pkg/CFR-2012-title26-vol2/pdf/CFR-2012-title26-vol2-sec1-163-1.pdf
Since you and she are both on the deed, you can each legally deduct the mortgage interest. Since it was paid out of a joint account, it's up to the two of you to decide what percentage each gets to deduct. Usually in these situations the split is 50/50.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
