Sorry to say that the abs you're probably looking for require years of consistent effort. It's a lifestyle, and lifestyles overtime affect the body and appearance. The appearance of strong flat defined abs/pelvis/obliques need to be built and maintained. A couple of sit-ups isn't gonna do it. Your whole body needs to understand the exercise; your veins and muscles connect from head to toe, so you need to do holistic exercises regularly. Recreational or competitive running and/or boxing, swimming/water sports, gymnastics, even just cartwheeling around the house and garden for an hour. Climbing (rock climbing, stair training, hell even tree climbing) strengthens the entire body and core muscles. Cross fit/aerobics mixed with acrobatics/high intensity is best. You can do this right in your gaff, star jumps mixed with frog jumps, mountain climbers and burpees and planks while using small weights. If you want sure then only eat a salad that day, but I wouldn't shy away from pizza and burgers and pasta still. Eat up.
Btw I don't agree with diet, unless you're currently overweight. I know petite and plus sized people with horrible diets but who have visible muscle and vein definition, because they stay active doing the above. You can eat healthy all you want but if you're wasting those nutrients by not exercising or staying active you're as good as eating maccas. You can eat greasy chips and that as well--as long as you stay active to burn some of that out.
I also don't believe that "genetics" are involved either, because people can grow and improve and end up looking different than their relatives or their own long time self.