Welcome to the GERD /Heartburn forum! Many members have hiatal hernias, so I'm sure you'll get other helpful responses from experienced members. You will be the best one to judge whether or not the sit-ups cause an increase in reflux episodes. You should definitely exercise before eating...a full stomach will most likely add to your chances for reflux.
If you can do sit ups comfortably, I wouldn't worry about it, but you should definitely check with your doctor to be sure.
Good luck finding answers to your questions!