My sister when to the doctors twice and they did a std test, pelvic exam, vagina swab, ultra sounds, a ct, etc. and it all came back normal. her symptoms are burning, discomfort, irritation when peeing, tenderness in lower abdomen, what could it be?

The lower genital symptoms -- burning and irritation with urination -- are typical for a vaginal yeast infection, which sometimes can be difficult to diagnose. Your sister should ask her doctors about it, including the possibility of trying an anti-yeast treatment, even if yeast hasn't been found. No harm will come from trying it. However, yeast can't explain low abdominal pain or tenderness.

These are too many tests for burning in urine. What about a urinalysis? If symptoms persists she should see a Urologist. In the meantime ask her to drink plenty of water. Hope she feels better.