You should consult an attorney to make sure this is on the up & up. And maybe a CPA too. The kind of questions you are asking seem like you don't know much and could get into big trouble.
As far as I understand, you must pay some kind of interest. Even between relatives, there is an IRS chart to look up for the minimum rate. Here's an article about it:
http://www.ehow.com/about_5097624_irs-imputed-interest-rules.html
Here's the chart from the IRS, looks like it comes out monthly:
http://www.irs.gov/app/picklist/list/federalRates.html
As far as I understand, If she is single (and it is her primary), she can claim up to 250K gain with no tax. But if she is just getting payments, the gain is not happening all at once... I don't understand how the IRS calculates this.
I know nothing of California capital gains laws, if any exist.
There's also a new rush of elder abuse laws enforcement- that includes financial dealings.