Yah ..  texan .. I had also invited my mum here in USA  and my condition and urs are same..   i am also in F1 status, I also wanted to seemy mum so she came here in tourist visa and she is with me now. I had send he the following doc.

1. My class schedule of 2008 (which showed all th fee that I had paid for entire year including summer)

2. An invitation letter (same as letter of status) written by International Advisor saying the student wants to invite his mum.

3. As you mentioned that you are sick and has health prob, so why dont you tell your doctor to write a letter stating that at this moment it would be goood for your health and comfort if your dad  is here. I told my doc to write that.

4. Back home, mu mum simply showed the bank balanc of 10 lakh, nothing more than that and they just asked my mum why she wants to visit.That was alll and it was just recently in january she had interview ...

gooood luck .. .. . .. . write me back if you need more info