Whenever my sister-in-law happens to meet any one of my relative by chance, she always makes sure she complains about me. I find it very disturbing as she does it even with my distant relatives who have no clue about anything and take potshots at me. What can I do?

Just stand up straight, face her and tell her what her problem is. Sorry to sound this way, but sometimes, you just need to behave with others the way they behave with you, so that the message reaches loud and clear. Tell her you have tolerated this behaviour enough, and if she doesn't stop, you will start telling others about this trait of hers. i am sure she will stop when she sees you retaliate.