First and foremost: Assuming she is a Muslim you have to be %100 sure if she is supporting herself with riba. (If your relative wasn't a Muslim then things would be differently). If you are not sure then you have no problem in eating. If you are sure:
You see, your friend, your father, your neighbor...everyone they are all working for someone, or with someone. Do you think all their money is pure?
I mean your father's employer does he make his money in a pure way? OK. What about the people who work with your father's employee? What about your friend's partner's client? You see where I'm going with this?
In general this is unavoidable but it doesn't mean that we should be careless.
Also do you know ALL her income is made through this way? I mean from when she was 10 years old until now, did she always make her income through riba? I doubt it. Do you know if she bought the food from all her non-halal income? Again I doubt you can be certain. I don't think she even separates her money this way. Like let me go buy food with halal-income but fix my car with non-halal-income. So because you can never successfully separate then you can eat.
So you are to accept it and assume that the money is coming from her non-haram way of income.
What you should never do yourself is to support yourself from riba. Others is not your problem.
Having that said my knowledge on this subject is 7/10 so do double check with an Imam you trust.