The answer to your question is that you do have some gift tax consequences related to this transaction, and that your sibling should file a Form 709 (federal gift tax return), reporting a total of $6,000 as a gift made to you. Please allow us to explain how we arrive that that figure.
Under estate law, joint tenancy is a special type of ownership by two or more persons of the same property. The individuals, who are called joint tenants, share co-equal ownership of the property and have equal, undivided, rights to keep or dispose of the property. Joint tenancy also creates a Right of Survivorship. This right provides that if any of the joint tenants dies, the remainder of the property is transferred to the survivors.
Treas. Reg. § 25.2511-1(h)(5)) explains that the transfer of money or property to another party (related to the taxpayer or not) constitutes a gift.
Because joint tenancy creates co-equal owners in the property, and there are exactly two of you, each of you essentially now own a 50% equity interest in the property. With an initial total cost (down payment) of $80,000, that means you each made an initial $40,000 as an equity investment.
However, you indicate that you contributed $20,000 of this $80,000 total amount, leaving some $20,000 that should be considered an effective "gift" to you on the part of your sibling, in order to equalize the investment at 50% / 50%.
That said, each taxpayer is allowed to give $14,000 per year (in 2016) to another person completely free of gift tax implications. Thus, $20,000 - $14,000 = $6,000 is the derived amount of the gift, to you, that becomes a reportable item on the part of your sibling.
Please keep in mind that this $6,000 is not actually a taxable sum in itself, as the $6,000 will simply count toward reducing you sibling's lifetime Unified Gift and Estate Tax allowance (which in 2016 is nearly $5.5 million). Nevertheless, there is still a reporting obligation, even in the absence of any taxes due, that legally should be met by filing Form 709.
Finally, please note that the gift tax return, Form 709, is not supported in the TurboTax software; and that your sibling would need to prepare it independently of TurboTax.
Thank you for asking this important question.
