I'm trying to understand the rules for stepped up basis on a house owned as joint tenants with right of survivorship.

Specifically, this is for joint tenants that are not married.  

Let's say that my mom added me as a joint tenant to her house, but I didn't pay anything for the house.  That was a gift above the annual exclusion, so she's supposed to file a gift tax return.  When she dies, here's what happens according to the book "Plan Your Estate" by D. Clifford (15th ed, p. 172):

To keep things simple, let's assume the value of my mom's estate, including the whole value of the house, is below the estate tax exemption, so we don't need to worry about gift taxes or estate taxes.

OK, now let's assume that I help my mom buy a house.  To keep it simple, let's say we buy it in cash, and I pay for 50% of the cost of the house.  When my mom dies and I inherit the house: (p. 171)

This is clearly a worse deal for me (assuming the house has appreciated).

In this case, wouldn't it be convenient if I were unable to prove that I paid for part of the house?  That way, it would be all considered part of my mom's estate, and I would receive a step up in basis on the entire house.

Is there something I'm missing here?