The phrase "to drop in" or "to drop in on" means to stop somewhere. In your sentence, "I want to drop in on my little sister in the afternoon", it means that you are going to visit her where she is at. A similar phrase, "to drop by" also works. "To drop by" can also mean more than visiting. For instance, if I say, "I need to drop by the store" it means I need to buy something there, and it probably won't take much time.

I think we often use "to come across as" to indicate that someone appears in a way that they don't intend. For example: She tries to be nice but comes across as arrogant. He means well, but he comes across as an asshole.