If your sister has dark skin and dark hair, and you have fair skin and fair hair, people might say that your looks are dissimilar or not alike.
Inside of dissimilar you find similar which means alike. By now, you've probably figured out that dis at the beginning of a word tells you the word means the opposite of whatever comes after. That rule holds here. Dissimilar is the opposite of similar. You've probably heard that you should not use double negatives. Well with dissimilar, it's okay to say something like, "Art is not dissimilar to music in that it is a creative activity. Here, "not dissimilar" means similar.