Can a mother of two under c - section, do sit up's as exercise? It's 1yr and 8 months after surgery.

Of course! I would highly recommend core strengthening exercises such as pilates.
Yes. In general, yes. However, if you're having any problems since the c-section, you should see your obstetrician.
may resume exercise. Even resume normal exercise of this point granted that there are no significant abdominal wall issues. That is if there is a hernia or a bulge you may want to discuss possible treatment with your physician prior to resuming situps and normal exercise.