My 13 year old brother who is 90 kg lifted up my sister who is 50. he felt a slight pain in his abdomen for few mins. he is paranoid about a hernia?

Muscle strain: It is more likely thst he strained the rectus abdominus muscle, and did not herniate his intestines through an abdominal wall defect. If the pain oersists beyond a few minutes, I woukd suggest rest and no heavy lifting for at least a week. If the pain is persistent, it would be a good idea to visit your doctor to make sure. From what you described, this does not appear to be a hernia.