According to IRS Publication 551, page 10, @Anonymous is correct. If the property was held as JTWROS and deceased father contributed 90% of the purchase price, 90% of its FMV at his death is includible in his estate, and is eligible for step-up in cost basis.
JTWROS with a spouse is considered a "qualified joint interest" and the FMV at a spouse's death is split 50/50 regardless of contribution.
https://www.irs.gov/pub/irs-pdf/p551.pdf
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
