Rudy and Liesel are like brother and sister in the fact that they spend a lot of time together; any time outside of their homes is spent in each other's company. As a result, they know each other really well, like a brother and sister do. They know each other's weaknesses and strengths. They get into trouble together, and also get each other out of trouble too. That is very much like siblings. Also, they never do hold hands, or kiss, or anything of that sort--so that makes their relationship familial also.
They are like lovers in the sense that they care deeply for one another, and would do anything for each other. Note how Rudy, skinny and starving, jumps into the freezing river in the middle of winter get Liesel's book. They care so much about each other that they would gladly give their lives, and all that they have for the other person. And, we know from the beginning of the book that Rudy loves Liesel and is constantly demanding a kiss. Liesel loves Rudy too, but just likes to torment him by not giving him a kiss.
Their relationship is a bit like a stranger's relationship only in the fact that they met as strangers, and come from different backgrounds. Liesel is an orphan, Rudy comes from a large, large family. Rudy is blond and tall, Liesel is dark-haired and small. They look like strangers. They also never get too deep in their conversations--Liesel keeps information from Rudy, for example, about Max hiding at her house. She treats Rudy more like a stranger in that respect.
They aren't like enemies much--they do fight occasionally, and are mean to each other. They also tend to get each other into sticky situations, like with the apple thieving, and book thieving; enemies like to put each other in compromised situations. But other than that, I wouldn't say they are like enemies at all.
The relationship between Rudy and Liesel is a strong, moving and dynamic one, and one that keeps the reader attached and involved in the story up until the very end. I hope that helped; good luck!