First of all, are you sure your boyfriend even does magic or has a family member who does? This may not be as far-fetched as some would think, since many cultures use magic quite frequently, curses too.
Number two, you do not have to believe in magic for it to work. That is a bunch of new age garbage. If a spell is cast properly with intent it will work regardless if the target believes in it - or even knows about it. So if you think there's a good chance there is actually a spell cast, then i'd take care of it.
Number three, the good thing is that simple curses are relatively easy to remove. Do the work and don't fret over it. if you constantly obsess about the spell and your grandma's health, then you'll be feeding the spell energy. Again, take care of it and then be at peace, confident that the spell is gone.
Number four, your work can be as complex or as simple as you can manage. ideally you would do cleansing work, followed by uncrossing or unjinxing work, followed by protection or blessing work. My first stop would be to research such spells at www.luckymojo.com. They have a fantastic database of real, proven, traditional spells plus a pretty excellent forum where you'll get solid advice with no judgement. I also have a collection of spells on my blog: http://ldygry.livejournal.com
I would start with removing absolutely anything that your ex gave you including letters, gifts, photos, etc. Throw them away. Next do a series of cleansing baths for you, coupled with a spiritual house cleaning. I like at least seven days worth of baths. You'll find instructions on how to take spiritual baths on both my site and lucky mojo.
I would try to incorporate something for your grandma. She probably wont be cool with doing spiritual baths herself, but there are sneaky ways to add some cleansing to her: add some cleansing oil to her shampoo, have her drink mint tea that you've blessed and prayed over, do some candlework for her, hide a mojo bag under her bed, etc. Again, do your research on lucky mojo. I have some healing spells on my blog too.
After your cleansing baths, or at the same time, find an uncrossing/unjinxing spell that you can do. These can be simple or complex. Research this online.
Following this work, you may want to also look into some blessing and/or protection work for both you and your grandma. What you've done is basically swept yourself clean spiritually. It's nice to add some blessing work to that clean slate.
If you think your ex is doing constant work, like regularly doing magic against you, then you will need to repeat your efforts on some sort of regular basis. Fortunately most people just aren't this crazy, but you would know best. In that case, i highly recommend doing some protection work and keeping that up.
Best of luck!