Modi has clarified that he is not seeking the details about the ailment and supports government of India paying for it. He has mentioned that he is interested only in her travel expense between 2004 and now and it is a valid question considering she is holding a public office and so she need to disclose the details of her travel.
Here is a snippet from Indian Express.
Standing by his questions, Modi said he had not asked about Sonias illness but about her foreign tours from 2004 (when the UPA came to power) to August 15, 2012. Soniajis illness came into the picture recently. But the questions (under RTI) were asked for three years. And they were only about her travel expenses. One should not make it an emotional issue. I myself tell the PM that the country should bear whatever expenses are needed for Soniajis treatment, its a human cause. But when a youth has filed an RTI and you take pride over the introduction of RTI, then please give a reply.
Sonia tours: BJP, Modi cash in on 1,880-cr error - Indian Express