I agree with soultax09 that JT means a 50/50 split so 50% gets stepped up. in addition, since dad paid for 90% but only ended up with 50%,he made a gift to you that should be(en) reported on a gift tax return. say price was $100 he paid $90 but ended up with $50 so the gift is $40. if the purchase was in 2019, then no return is required if the gift was less than $15,000. the executor would need to file. the gift to you doesn't get stepped up.
what if the purchase was a long time ago and a return wasn't filed. i'm guessing that the IRS would do nothing as long as no tax was due.
you are free to consult with an attorney to see if we're missing something
