Hello again, sh62365:
Joint Tenancy ownership of property can indeed be complicated, as experienced accountants and attorneys will be quick to tell you.
But it seems like you and your sibling have a pretty good grasp of what's going on now, and you know the necessary tax forms to file. In my opinion as a CPA, I don't believe that the IRS will have any problems with your arrangement whatsoever, in terms of one sibling taking the entire deduction for mortgage and property taxes, so long as the gift tax reporting rules explained elsewhere in this answer are adhered to. In fact, after you do this for more than one year, preparing and filing Form 709 should become fairly routine (as it will be substantially the same disclosure each year, assuming your arrangement continues).
Good luck!
