Can you tell me if she worked and paid into her home countries social security system?
and what country is that.
The U.S. has some recipricol social security tax agreements with countries, known as totalization agreements. i would like to see if the U.s. has a totalization agreement with her country; she might be able to get benefits under SS on her own.
in the mean time, to answer your initial question:
Use the income of the individual you are applying for. You will not be using family income. The reason is that eligibility and payments are based on the individual Modified Adjusted Gross Income. Since she is being claimed by you, she will loose the personal exemption, which means her MAGI will be more than it would be if she were claiming herself.
you state she has no income, but she does have some investment income.
The minimum premium is 25%.
You can continue to claim her as a dependent on your income tax provided that she does not provide for more than 50% of her own support, under the other conditions you have stated.