Unless you have the means to prove otherwise (and I assume you do) then the split is 50/50. Otherwise if you can prove it, then it's 90/10. Also understand that just because one owner contributed 90%, does not always mean they own 90%. So don't be surprised if challenged by the IRS on your 90/10 claim.
There is no gift tax. Your deceased father did not in any way, form or fashion "give" you his share. With a JTWROS ownership, you "automatically" inherited his share and the inheritance was outside of any last will and testament he may have left.
Your step-up in cost basis will be the appropriate percentage of the FMV on the date of his passing, and "NOT" the date you acquired full ownership of the property.
You should be receiving a K-1 from his 1041 Estate return showing the inheritance passing of his ownership to you. There should be no need to enter the K-1 in your tax return, but there "could" be. So if unsure, just ask once you receive the K-1 and can see what all is on it. (For example, if you inherit a tax-deferred retirement account, it "could" be taxable to you, depending on how you handle it.)
