I am not a fan of traditional sit-ups or leg lowers (especially double leg lowers) as often people perform these loaded exercises incorrectly or with poor form, which can lead to pain and injury, especially in the lower back.
Instead I prefer to teach my clients how to plank. The plank can be a great exercise to build strength around the muscles of the abdomen. It also helps develop strength in the shoulders, arms, and glutes. The plank is a simple, safe and effective core exercise if done correctly.
If you can maintain a neutral lumbar spine (see picture below) and are not hanging into extension then you should be able to include a plank in your exercise program.