You might get annoyed if people keep saying there is a similarity between you and your older sister, particularly if you're tired of being compared to her. A similarity is a sameness or alikeness.
When you are comparing two things — physical objects, ideas, or experiences — you often look at their similarities and their differences. Difference is the opposite of similarity. Both squares and rectangles have four sides, that is a similarity between them. Just because two things share similarities doesn't mean they are the same.